The problem is either with the operating system or with the hard drive itself.
Your first step should be to check if the hard disk drive is being detected in the BIOS setup. If the BIOS setup shows that the hard drive is connected, you may want to repair the operating system using the OS installation/bootable disc. In same cases, clean installation of the operating system may also be required.
The process to repair or clean install the operating system depends on the OS that you were using and/or the one that you want to use and on which laptop/desktop. If you could provide some more details about the computer, it would be easy to give you step-by-step instructions.