|A ps/2 mouse will only work if it is connected at bootup. It is not like usb which you can plug and unplug.|
Before you do a system restore try another mouse or at the least test the mouse on another system to assure its is good.
You don't mention optical so I have to ask if you have taken the roller ball out and cleaned the two roller contacts?
Mice are cheap. Get a new one if you have any doubt about this being a working one.