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This vexed me for a bit and I'm posting here in case it helps anyone else. I'm on 17.10 which is very new (released yesterday), so this may yet get fixed.

Mouse was working, then next day was not working once logged in, but was working on the gdm3 greeter screen.

I'm using a xorg not wayland session, if that matters.

Turned out it libinput's libinput Accel Speed setting was at -1 which means, erm, no accelleration and therefore no movement!

A. If you have another mouse/touchpad

Go to mouse settings and ramp up the mouse speed with the slider. Mine was fully left. Is this a feature or a bug? Probably the latter.

B. If you don't have another mouse/touchpad

Start a terminal.

$ xinput list

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

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

⎜   ↳ MOSART Semi. 2.4G Wireless Mouse          id=11   [slave  pointer  (2)]

⎜   ↳ Microsoft Natural® Ergonomic Keyboard 4000    id=14   [slave  pointer  (2)]

⎜   ↳ ELAN Touchscreen                          id=15   [slave  pointer  (2)]

⎜   ↳ DLL06E4:01 06CB:7A13 Touchpad             id=16   [slave  pointer  (2)]

⎜   ↳ SynPS/2 Synaptics TouchPad                id=20   [slave  pointer  (2)]

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

Look for the thing that might be your mouse. e.g. mine is MOSART Semi. 2.4G Wireless Mouse.

Next check it's configuration:

$ xinput list-props 'MOSART Semi. 2.4G Wireless Mouse'

Device 'MOSART Semi. 2.4G Wireless Mouse':

    Device Enabled (143):   1

    Coordinate Transformation Matrix (145): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000

    libinput Accel Speed (280): -1.000000

    libinput Accel Speed Default (281): 0.000000

    libinput Accel Profiles Available (282):    1, 1

    libinput Accel Profile Enabled (283):   0, 1

    libinput Accel Profile Enabled Default (284):   1, 0

    libinput Natural Scrolling Enabled (285):   0

    libinput Natural Scrolling Enabled Default (286):   0

    libinput Send Events Modes Available (265): 1, 0

    libinput Send Events Mode Enabled (266):    0, 0

    libinput Send Events Mode Enabled Default (267):    0, 0

    libinput Left Handed Enabled (287): 0

    libinput Left Handed Enabled Default (288): 0

    libinput Scroll Methods Available (289):    0, 0, 1

    libinput Scroll Method Enabled (290):   0, 0, 0

    libinput Scroll Method Enabled Default (291):   0, 0, 0

    libinput Button Scrolling Button (292): 2

    libinput Button Scrolling Button Default (293): 2

    libinput Middle Emulation Enabled (294):    0

    libinput Middle Emulation Enabled Default (295):    0

    Device Node (268):  "/dev/input/event8"

    Device Product ID (269):    1578, 16642

    libinput Drag Lock Buttons (296):   <no items>

    libinput Horizontal Scroll Enabled (297):   1

You'll see on mine libinput Accel Speed (280): -1.000000 which is the trouble.

You can set that to something more sensible with

$ xinput set-prop 'MOSART Semi. 2.4G Wireless Mouse' 

'libinput Accel Speed' 0

But you'll have to use the name of your own mouse, obvs.

Just to thwart the physicists, an "accelleration speed" of 0 is not no accelleration. -1 is no accelleration. Anyway, once you've done this give the mouse a wiggle and see if it moves.

Probably setting it this way does not stick between sessions so you should probably now use your newly freed mouse to do (A), which I think does set it permanently.

I had the same problem and tried the suggestions here to no avail.

I solved it by opening the mouse and adjusting the direction of the emitter of the optical light. It had moved when the mouse fell and so was no longer directing the light downwards. The re-orientation of the "optical emitter" solved the problem.

I hope this helps someone.