Have you changed the keyboard since you were last able to get into BIOS Setup?
If yes, and your current keyboard is a USB one, try plugging in a PS/2 keyboard into the computer's PS/2 keyboard port. There should be a pair of PS/2 ports at the back, one for a keyboard and one for a mouse.
The two PS/2 ports are physically identical but not interchangeable. The one for a keyboard is usually the one that's nearest to the edge of the case and is colour-coded lilac or purple.