Well, the bios looks to the hard drive for the OS, and if it can't see the HDD, that suggests a problem there. But have you changed anything in the bios - is the machine looking to the DVD drive or similar?
If it got as far as the logon screen, not a hardware fault. Even if it only booted, I would suspect software but could be media left in the drive (floppy or DVD). Otherwise, your C:\ drive has become non-bootable or your OS has been damaged. Try running an OS repair.