Also - if you had unplugged the keyboard prior to it playing up - have you checked the pins inside the PS2 plug to make sure none have been bent when it was plugged back in ??
The easiest way to check if it's the PC or the keyboard at fault, is to get holf of a keyboard that you know works - from another PC, or even buy a new one - you van get a basic keyboard these days for only a couple of pounds / dollars / euros - or try yours on another PC that is known to work with a PS2 keyboard.
If everything else on your PC seems to be working, and the keyboard lights aren't coming on, then a faulty keyboard is the most likely.
A USB keyboard should work but you may find that you can't get into the BIOS using it, as the USB drivers for it won't be loaded by Windows until after the timeout for pressing "Del" to enter the BIOS.
Good luck
