Hard drives are very fragile; a minor bump could cause the read head to come loose and become damaged. Run a ChkDsk to see if your hard drives have any errors first:
To run a chkdsk:
- Open "My Computer"
- Right Click the drive in question
- Click "Properties"
- Select the "Tools" tab
- Click [Check Now] in the Error check section
If you find problems with the hard drive, you'll have to replace it. Hopefully your lappy is still under warranty. If it is, just fib to the company that the drive became loose somehow, they'll usually have you send it in to be repaired.
Hope this helps,