Go in to your BIOS and see if your system still sees the the boot hard drives. The one that your OS was installed on if you have more than one drive.
If your system still sees the hard drive, one can just boot up from the OS disk and run a repair. This should resolve the problem. This will not affect any of your personal files.
IF your system does not see the hard drive, remove it and try to install it into another PC and see if that system sees the drive.