Dell optiplex BIOS access key is either F2 or Del. If you can't access it by using those keys, your CMOS must have gone corrupt. You need to flash it. Just remove the CMOS for some seconds and put it back, than you will be able to enter setup.
Hope this helps.
Sorry about the delay, was away for a while.
CMOS is a flat battery located in your motherboard. Flashing CMOS means just removing it for sometime and than placing it again.