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I am new to Linux, hence my ignorance, my apology. Touchpad on my Dell Latitude5490 with Ubuntu 16.04 is not responding anymore, but USB mouse does. I tried solutions of many similar posts, without success. In fine, I tried everything as listed in this post: Enable SynPS/2 Synaptics TouchPad in 14.04, but nothing works.

Actually, when using command xinput -list I don't see anything that refers to touchpad, here the output:

    ⎡ Virtual core pointer                      id=2    [master pointer  (3)]

⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]

⎜   ↳ PixArt Dell MS116 USB Optical Mouse       id=9    [slave  pointer  (2)]

⎣ Virtual core keyboard                     id=3    [master keyboard (2)]

    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]

    ↳ Power Button                              id=6    [slave  keyboard (3)]

    ↳ Power Button                              id=7    [slave  keyboard (3)]

    ↳ Sleep Button                              id=8    [slave  keyboard (3)]

    ↳ AT Translated Set 2 keyboard              id=10   [slave  keyboard (3)]

In system settings, "Mouse and Touchpad", I indeed have nothing that refers to the touchpad. It happened when I turned the computer on today, after turning on while it was not used for a while just yesterday. I used updet/upgrade commands and then faced the problem listed here Unable to lock the administration directory (/var/lib/dpkg/) is another process using it?, solved (at least I hope) by killing running commands. Any idea how to solve that and, to avoid repeating the same mistake, how did that happen?

EDIT: I don't now if there is any link, but when I close the computer putting it on hold, and then try to reaccess it, despite being on, screen never turns on too and I can't access to the session.

Thanks,
B.

SOLUTION: The solution I found was to delete the new kernel and use the previous one. To do so, for neophyte as I am, you need to restart the computer, press escape (with unbutu unity at least), when the computer is turning on, to have access to different options. Select the first option with advanced unbutu features, and select the previous version of the kernel (in my case, the fourth row). Then you need to delete the most recent version of the kernel you were using. You can see what is this version either by memorising it while you pressed the escape button at first, or, if you are already using the older kernel version, by entering the command "dpkg -l | grep linux-image", the later version being the one to remove. To do so you then use the common command "sudo apt-get purge linux-mage-XXXX the version you want to delete XXX.
Hoping that helps, please correct me if that is actually absolutely not to do !
Best,
B.