Dell BIOS can't detect harddrive

Just have a question on what can cause a harddrive not to be detected in BIOS. Here's the scenario:

Have a computer at work had a hard drive failure. IT still has not fixed it. Use other co- workers computer when not there. Co- worker does not like ,I'm guessing, other people using his PC, so when I came in one day would not load applications I need for work. Discovered network cable was removed. Took cable off my broken PC all good. Next day went to turn on computer getting message no hard drive detected. Oh boy! I'm not to savy with computers and their workings but hit F2 (Dell) for BIOS screen, says hard drive not detected, drive ID# missing. Strange I thought.

My question is how does someone do this? Computer is Dell optiplex 775, harddrive sata-0, cd sata-1, boot sequence fine. 80 gig harddrive with OS XP on it. How does someone remove harddrive from BIOS? IT is autodetected by BIOS. Weird is when I gave up and used another computer the next day that computer was working and harddrive was installed again. If anybody body has any idea on how to accomplish this I would appreciate it. Can see person opening case to remove sata cable because someone is using their computer, be it company computer I might add.

Thanks,
Mar

They could be just opening the case and unpluging the data cable to the drive. Dell cases open very easy. There are some real jerks out there.