The harddrive may have become dislodged from its contacts.
Try: Remove all power including the big battery and then hold in the power button for about 20 secs (to drain any residual power and clear volatile memory). Then take out the harddrive and then put back in. (There may be a screw to undo before the harddrive can be removed)
Now attach power and boot up. Hopefully the BIOS can now see the harddrive and boot into Windows.