You will need to cold boot the server. At the hardware splash screen, immediately after POST (the beep), press the correct key to access the BIOS. This will usually be Delete, F1, F2, or F11, but may be different depending on the hardware manufacturer and BIOS version.
Setting a password on the BIOS is primarily a security measure that keeps the wrong people from accessing and altering the basic hardware settings. The primary disadvantage is that, on some systems, the BIOS password can also set the boot password. What this means is that a password must be entered before the machine will boot. If the server loses power or must be rebooted, somebody will have to enter the password [u]at the server[/u] before the server can reboot, which is not a good thing for remote servers. It is also possible to reset the password if you have physical access to the machine.
Most sites I've been to rely on physical security (locked doors, access control, etc.) to prevent BIOS access to their servers.