What kind of computer is it? Does the screen come on?
Hopefully it's just the hard drive that has come loose or was damaged, and the computer can't see anything to boot from. If you have your installation discs, try booting from the install discs to see if the rest of the computer still works. If it does, see if the disc can see the hard drive. If it can, either the hard drive is damaged, or something else has happened that's harder to diagnose over the internet. If it can't see the hard drive, it might have just gotten knocked loose of its connection.
In that case, open up your laptop company's website and find instructions for replacing the hard drive. Follow the instructions, open up the computer, and check whether the hard drive is securely installed.