Boot into BIOS. Look at the boot order list. Is your hard drive there?
Either the BIOS is not seeing the drive or the drive is no longer bootable.
If the BIOS is not seeing the drive POST here. There are one or two things to try before you replace the drive.
If the drive is not bootable then POST here too. It might be as simple as rebuilding teh master boot record (easy) or resetting the SATA adapter mode (easy) or it might mean reinstalling windows.