The mouse driver probably got changed somehow. I suggest you boot in runlevel 3 (text mode), and check your /etc/X11/XF86Config-4 or equivalent file for the section about the mouse.
If the driver is "PS/2", try "IMPS/2", and the other way around. If the driver is neither, then try them as a start. Before the end of the week, I'll have a page, that *might* help you there:
http://yves.gablin.club.fr/linux/ind...e=config/mouse
but it's not there, yet... sorry, I'll try and hurry.
Yves.