If your computer doesn't find a working boot record on the harddisk or elsewhere, it asks you to insert a diskette with a bootable system. This means your operating system can't be found for one of many reasons - disk failure, loose cable, done something terribly wrong etc... Try to remember what you did / what happened before.
The first thing I would do now is to check if the HDD is still recognized by the BIOS. If yes, download a health check tool from the HDD's manufacturer website and check if the disk is ok. If the BIOS doesn't see the disk, check its cable and connectors.