hmm ,
unplug the mouse and do a restart, then plug it in to see it it works after that?

try keyboard short cuts??
http://support.microsoft.com/kb/126449

if this just happened recently and a few reboots don't fix it you could do a system restore
or

remove the mouse driver and then reboot and let it reinstall the mouse hardware.

it is a optical mouse or a roller ball - it could be the mouse is dirty or broken...