Two main things are a failed drive or simply a dryed out flat ide ribbon cable needing replacement. If you jarred the case any and the plus came out a little that would cause the loss of connection there since no signal would reach the controller card on the drive itself. Inspect and even try a different cable before assuming a failed drive would be the first idea on this.
One place to look right away when any drive isn't seen in the MyComputer and Windows Explorer is the Disk Management tool found in "Control Panel>Administrative Tools>Computer Management>Storage". Or you can simply reboot the system and look at the post screen as the drives installed are tested there. That requires that the board or system logo(HP, Dell, Gateway, et.) is first disabled in the advanced area of the bios for the quick look there to see if the drive is detected and working.
One other option is to go into the device manager and open the cd/dvd drives section to see if the drive is listed there. Right click on that and choose the uninstall option to have Windows perform a new detection of it. If the drive is still good simply have Windows automatically reinstall it's own drivers for it.