Hmm ... do you share that computer with someone? Did she set up a bios password and didn't mention it to you? That's what it sounds like -- does it ask for the password before looking for a drive to boot from? Do the lights on any of the drives light up when you turn on the computer, or does it just sit there and ask for the password?
If that's it, the cure is to open the case and take out the CMOS battery for 8 minutes. That makes the CMOS forget everything, so you can plug the battery into the motherboard again after 8 minutes. It will have forgotten all about the password, and it will also have forgotten the time and date and anything else you've changed from the default settings, but at least it'll boot up.
Sometimes people mess around with the CMOS and set a password without intending to, and sometimes one person sets it deliberately and goes off somewhere and forgets to tell the other person who uses that computer.
It's also possible to set a password in software but that's different; then the computer will read the hard drive and boot up before asking for the password. There's often a way to defeat a software login too....