There are several "things" which can cause your computer not to boot.
Installing a program (consider starting it up in Safe Mode and uninstalling the application using the Add / Remove Programs Control Panel.)
It could be a virus (LikeW32/Kriz.3863) which have a CMOS overwriting procedure to erase date and time, and the type of hard disk the computer uses.
Another possible scenario that can cause trouble is a hardware conflict or driver issue.
... It can be that the power supply not providing the proper voltage.
.......
But you know what? ... Turn the PC on and count how many beeps your BIOS is making...
Check this ...
http://www.tcwo.com/support/new/beeps.htm