There could be a number of problems from OS failure to memory failure, etc... If you noticed any "memory error" messages prior to this mishap, you might simply need to replace your memory chip(s). Otherwise, if you're using Windows XP, you might also try to do a system restore by pressing one of the Function keys (F2-F12) designated... They vary from system to system, so if you don't know which key it is, you may have to check your manual or experiment with each button after each reboot.
Hope this helps.
