Many used computers are being sold today. Often former owners place a password in the BIOS to protect the BIOS settings from being changed by others using their computers. As humans go, these passwords are lost. Consequently, this forgotten password can not only block new owners from making necessary BIOS changes, but sometimes can block the new owners from even booting up the computer.
To get around this problem, you have to reset the BIOS settings, which are stored in a special type of memory - called CMOS - on the motherboard. If you have the PC's motherboard manual (or can find it online), look up something called the clear CMOS jumper - a tiny plastic connector bridging a set of pins. Follow the procedure in the manual to reset it; typically, you will shut down the PC, remove the power cord, shift the jumper to a different pair of pins, plug in and restart, then repeat the process, returning the jumper to its original state. The procedure varies by board, though (and can have motherboard-mortal consequences if you deviate from it), so do not just wing it - look it up!
A quick-and-dirty alternative, it you can not locate the jumper or manual, is to yank the CMOS battery. Usually a coin cell (similar to an oversize watch battery), it feeds a trickle of power to the CMOS memory to preserve the data it holds when the PC is off. With the power detached as above, remove the CMOS battery for about 10 minutes, replace it, reconnect everything, and boot up. Since you have the battery out anyway, it would be a good idea to purchase a new one (available from almost every place small batterys are sold). The PC should have reverted to its factory BIOS settings, minus the BIOS password.