There's a few ways to deal with this - not all works for all systems though.
Try read your Motherboard manual for any reference to a "CMOS/BIOS resetting jumper" (Or something along those lines), it's basicly a small piece of plastic with some metal inside, where it is and where to put it in order to reset it, should be in the manual if available.
If not, then try this (this is what I do):
1. Unplug the cable for the power supply unit or shut it off, whatever works best for you.
2. Open the case to your computer, on the motherboard there should be a lithtech battery mounted on it (it's a battery at the size of a coin, it's often silverish coloured).
3. See if you can find a way to plug the battery out.
4. Leave it out for a good 15 secounds (~5 should be enough, but why not be sure?)
5. Plug it back in (should be an easy job - at least easier than getting it out)
6. Boot your computer back up - now it should be working
*Note - that this doesn't neccessarily work on all systems, especially not newer motherboards. If it asks for your password, try just pressing Enter (leaving the field blank), or try with "admin".
Rene 'zitrez', October 2006