When i try to start my computer it says that my hard disk is missing

Chances are your hard drive has died, i hope you have backups of anything important.
It could be possible that you have a corrupt boot sector but it is best to enter setup in your bios and see if your hard drive shows up if not you need a new hard drive.

Before replacing the HDD try re-seating the connectors. Also try different cables and different MB ports.