My question is, how can I find boot log from previous system boot attempt?

Today when first powering on my PC, boot process stopped on Ubuntu logo, when I pressed Esc I have seen several lines containing some kernel error and restart required at the bottom, so I pressed Ctrl+ALt+Del and next boot went OK without problems.

I have trouble finding messages from the screen I have seen during the first unsuccessful boot. Should I have taken picture to my phone?

/var/log/boot is there but empty, I searched kern.log and syslog for strings I remembered with today's date like error but found nothing familiar to what I have seen on previous boot screen.

$ journalctl -b -1 gives me only kernel messages during boot, I can find that elsewhere too, and they are not what was appearing on screen during boot, journalctl is useless for me, I am looking for messages appearing on screen during boot time.

For now, only option is take a photo of write the message on paper.

Reported as a bug that's an undocumented feature

There is a bug report filed on this topic. Because rsyslog already maintains multiple boot journals in /var/log/syslog and syslog.1, .2.gz, .3.gz... syslog.7.gz the developers felt keeping extra journalctl logs would waste disk space.

The bug report states on January 3, 2018 that for new installs rsyslog will no longer be the default and that journalctl will keep multiple boot data logs.

Create multiple boot logs without reinstalling Ubuntu

Most of us won't do a new install so to enable multiple journalctl boot logs in which case we can use:

$ sudo mkdir -p /var/log/journal

$ sudo systemd-tmpfiles --create --prefix /var/log/journal

Cannot set file attribute for '/var/log/journal', value=0x00800000, mask=0x00800000: Operation not supported

According to this github report the warning message "Cannot set file attribute" can be ignored.

Optional persistent storage setting

After using previous boot logging for many months I've discovered another option that can be set in /etc/systemd/journald.conf:

Storage=

Controls where to store journal data. One of "volatile", "persistent", "auto" and "none". If "volatile", journal log data will
be stored only in memory, i.e. below the /run/log/journal hierarchy
(which is created if needed). If "persistent", data will be stored
preferably on disk, i.e. below the /var/log/journal hierarchy (which
is created if needed), with a fallback to /run/log/journal (which is
created if needed), during early boot and if the disk is not writable.
"auto" is similar to "persistent" but the directory /var/log/journal
is not created if needed, so that its existence controls where log
data goes. "none" turns off all storage, all log data received will be
dropped. Forwarding to other targets, such as the console, the kernel
log buffer, or a syslog socket will still work however. Defaults to
"auto".

In a nutshell remove the comment and revise the line to:

storage=persistent

Display list of previous boots

$ journalctl --list-boots

-15 58a9e56135564cd8a52d547b19e76bf5 Fri 2018-02-02 18:34:35 MST—Fri 2018-02-02 23:07:14 M

-14 3514e056440341b1b6e5f03d109681bc Sat 2018-02-03 06:05:12 MST—Sat 2018-02-03 08:07:44 M

-13 0d1a32dc275348589f5ecdc72180c018 Sat 2018-02-03 08:08:05 MST—Sat 2018-02-03 08:08:34 M

-12 74159b593f3a401589ee6bd78e31684b Sat 2018-02-03 08:08:51 MST—Sun 2018-02-04 08:32:09 M

-11 4b394a9aad584ab2bfabe3b77eeed78f Sun 2018-02-04 08:32:26 MST—Mon 2018-02-05 16:54:02 M

-10 8e461ed2593c4fd896ca3b71eb3c0fba Mon 2018-02-05 16:54:34 MST—Tue 2018-02-06 03:54:30 M

 -9 ec7ba0e4dfe241c0b9c978d278fcca6d Tue 2018-02-06 03:54:47 MST—Tue 2018-02-06 16:25:02 M

 -8 b5c110267c214c38b63d0a367197d118 Tue 2018-02-06 16:25:19 MST—Thu 2018-02-08 16:49:03 M

 -7 75c3b117ac6a4de984dc3ced15edb7f8 Thu 2018-02-08 16:49:22 MST—Fri 2018-02-09 03:51:09 M

 -6 7338bd1007bc42dda5c8667eeefe1a59 Fri 2018-02-09 03:51:26 MST—Fri 2018-02-09 16:55:52 M

 -5 4b6cd0121327454ca3db035c7ed42df6 Fri 2018-02-09 16:56:09 MST—Sat 2018-02-10 07:55:14 M

 -4 0d56207f9ec0405ca3a3fd638334de2f Sat 2018-02-10 07:55:32 MST—Mon 2018-02-12 22:16:05 M

 -3 0f230cc546fd4aec8f5233e0074ab3e1 Tue 2018-02-13 03:57:20 MST—Wed 2018-02-14 22:58:56 M

 -2 c0d2c0141dd840cbab75d3c2254f8781 Wed 2018-02-14 22:59:13 MST—Sat 2018-02-17 22:46:14 M

 -1 aafb2573a6374e019a7165cb8eee74a0 Sun 2018-02-18 06:02:03 MST—Mon 2018-02-19 04:16:36 M

  0 8462f1969c6f4d61973e7e245014b846 Mon 2018-02-19 04:16:53 MST—Tue 2018-02-20 18:51:42 M

Display last boot log

$ journalctl -b-1

-- Logs begin at Fri 2018-02-02 18:34:35 MST, end at Thu 2018-03-01 16:43:25 MST. --

Feb 28 20:03:15 alien systemd-journald[290]: Runtime journal (/run/log/journal/) is 8.0M, 

Feb 28 20:03:15 alien kernel: Linux version 4.14.23-041423-generic (kernel@kathleen) (gcc 

Feb 28 20:03:15 alien kernel: Command line: BOOT_IMAGE=/boot/vmlinuz-4.14.23-041423-generi

Feb 28 20:03:15 alien kernel: KERNEL supported cpus:

Feb 28 20:03:15 alien kernel:   Intel GenuineIntel

Feb 28 20:03:15 alien kernel:   AMD AuthenticAMD

Feb 28 20:03:15 alien kernel:   Centaur CentaurHauls

Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point

Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'

Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'

Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registe

Feb 28 20:03:15 alien kernel: x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'

Feb 28 20:03:15 alien kernel: x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256

Feb 28 20:03:15 alien kernel: x86/fpu: xstate_offset[3]:  832, xstate_sizes[3]:   64

Feb 28 20:03:15 alien kernel: x86/fpu: xstate_offset[4]:  896, xstate_sizes[4]:   64

Feb 28 20:03:15 alien kernel: x86/fpu: Enabled xstate features 0x1f, context size is 960 b

Feb 28 20:03:15 alien kernel: e820: BIOS-provided physical RAM map:

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000000000-0x0000000000057fff] usabl

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000058000-0x0000000000058fff] reser

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000059000-0x000000000009dfff] usabl

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x000000000009e000-0x000000000009ffff] reser

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000000100000-0x0000000030a5ffff] usabl

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a60000-0x0000000030a71fff] reser

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a72000-0x0000000030a89fff] usabl

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a8a000-0x0000000030a8afff] ACPI 

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030a8b000-0x0000000030ad4fff] reser

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030ad5000-0x0000000030b2dfff] usabl

Feb 28 20:03:15 alien kernel: BIOS-e820: [mem 0x0000000030b2e000-0x0000000031099fff] reser

lines 1-29

Pay close attention to the parameter -b-1 it is different than other references you may see.

I had the same issue, and apparently found the answer on the #ubuntu irc-channel.

For whatsoever reason, I was missing the folder /var/log/journal group-accessible to systemd-journal.

After adding the folder, I was able to see logs of previous boots via $ journalctl -b1

The steps to accomplish the solution from the top answer here, from the man page for systemd-journald:

mkdir -p /var/log/journal

systemd-tmpfiles --create --prefix /var/log/journal

systemctl restart systemd-journald

I did this as su

The answer can be found in man journald.conf, specifically the option Storage=:

Controls where to store journal data. One of "volatile", "persistent", "auto" and "none". [...] "auto" is similar to "persistent" but the directory /var/log/journal is not created if needed, so that its existence controls where log data goes. [...] Defaults to "auto".

Please keep in mind that there is no need for log rotation or similar techniques that were common with the old syslog daemon. The journal file is by default configured to grow to a certain size and old log entries are automatically deleted when the journal file grows too large.

On my system this size is currently configured as 120MB, you can adjust it in /etc/systemd/journald.conf for the systemd-journald.service unit.

Use journalctl -bX where x is the boot you refer to, so -b0 is your actual boot and -b-1 the boot before (which only works if you have the folder /var/log/journal belonging to group 'systemd-journal' present). Cant tell you how far exactly you can go but those two for sure.

List available boots with

journalctl --list-boots