I accidentally disabled my touchpad. This is the way I found to re-enable it.
Press the "Windows key" to open the start menu. Type "terminal" and enter, to open the command line.
Find the 'id' of your touchpad. For me it looks like this:
Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ TPPS/2 IBM TrackPoint id=12 [slave pointer (2)]
⎜ ↳ SynPS/2 Synaptics TouchPad id=11 [slave pointer (2)]
My touchpad has id = 11.
xinput set-prop 11 "Device Enabled" 1
(but replace 11 with whatever id your touchpad had)