Touchpad suddenly stopped working

I recommend you use " touchpad-indicator" - an indicator applet for easily managing your touchpad. Installation instructions are given in this link.

http://www.webupd8.org/2011/02/touchpad-indicator-now-automatically.html

Perhaps, switching to the linux console (say, Ctrl-Alt-F1) and back to X (Ctrl-Alt-F7) will make the touchpad work again. (This is a workaround around the bug, wothout a good fix and explanation.) -- https://bugs.launchpad.net/ubuntu/+source/linux-ac100/+bug/1035208/comments/15

Try restarting your mouse or touch pad using xinput. First you should know the id of your device, for doing so type in a terminal:

xinput list

A list with input devices will appear, notice the id of your target device, then type

xinput --disable [id]
xinput --enable [id]

replacing [id] with the device ID you want to restart. It should work now.