When the computer is restarting, you'll need to press a key to enter the bios setup. More often than not, it's the DEL key, Compaqs F 10, other computers F 1 or F 2.
If neither of these work, post back the model/brand of your computer and someone can assist you further.
If one of these combinations work, once you are in the bios setup, look for information regarding the boot order of your devices, A: should be first: C: second (or floppy, first - hard drive, second ..... wording may vary). Also look for IDE/Hard drive information. If there is a key to recognize the drives, press that one.....save changes, exit and try starting again.
By the way, what's the history of the computer? Did it start correctly before? If so, what's changed (or what has happened) since the last successful start-up?