Look through the various sections of your BIOS, but don't change anything.
When you see something like 'SATA Primary' or 'IDE Master' or 'SATA Primary Master' or 'Harddrive' or something similar, stop there.
It might have the size of the harddrive also, eg: 500GB.
If you see something like the above but next to all of them there is a 'Not Detected' message, then either your harddrive is disconnected or there is something wrong with it.
Also, there might be something in the Slave sections, but this is normally your CD/DVD drive and it normally says too.
Edit: Thanks for the best solution!