|You should never, ever move a computer while it is switched on and running.|
Either the data or power cable may have become dislodged at the back of the hard drive.
Disconnect the PC from the wall socket.
Remove the side panel that is furthest away from the motherboard.
Check both cables that connect to the drive and make sure they haven't come loose.
If cables are okay, then the hard drive may have been damaged. It should never be moved while it's in use. Test it with SeaTools for DOS. If it fails the test you will have to buy a new hard drive. "SeaTools for DOS" is on the "downloads" link on this page: http://www.seagate.com/support/down...
SeaTools for DOS runs only from a bootable CD. To create the CD use "IMGBurn" from here: http://filehippo.com/download_imgburn/