BIOS (Basic Input Output System), Is the first piece of software loaded when you boot your PC. It has all the softwares necessary to control the monitor, hard drive, keyboard, ect...
The BIOS code is stored on a CMOS (complementary metal oxide semiconductor) chip on your motherboard, but because RAM is faster than ROM, the BIOS is copied from the CMOS to the RAM when the computer is booted. This is called shadowing.
There are different BIOS from different companies. The most used BIOS are Phoenix and Amibios.
How to Reset?
Boot your computer and as soon as you hear a beep or see the "Press DEL to enter setup" start pressing the "DEL" button on your keyboard a few times.
As soon as you press the "DEL" button, The BIOS Setup Utility launches.
The above picture is of the Phoenix BIOS Setup Utility. Notice on the right hand side that there are a few options to load the default settings. In the rabove situation you should go with the optimized default and then in case it doesn't work you should fall back on the fail safe default.
If your BIOS is really damaged, and that it even prevents you from accessing the setup utility tool, then you will need to do that manually by opening the computer case. Don't worry it is very easy.
Reset BIOS with the Motherboard Jumper.
Get your motherboard's user manual and look for the motherboard layout diagram. Look for the BIOS jumpers.
There are three pins, and the jumper connects only two of them at any time. You will need to move the jumper to the next pin as showed in the picture below.
Leave it in this position for about ten seconds to erase all your settings and revert to the default.
Just put the jumper back where it was and you are good to go.
To update your BIOS.
If you BIOS is corrupted for whatever reason, you can always re-install it.
Go to your computer manufacturer's site and look for the BIOS update. You'll find either a floppy or a CD version. There are different ways to do that depending on the manufacturer, so I can't just write how to do it. You'll have to read you manufacturer's Guideline to update your BIOS.
But basically, you'll have to reboot with the rebootable floppy or CD and this should update your BIOS.