Answer to: Mac won't start up after being knocked over...Check to make sure that the cables are still plugged into the motherboard and hard drive. If you look just under the hard drive bays you should see where the sata cables plug into the board. Make sure that the hard drive is spinning up as well. You can turn on the tower with the outside, aluminum cover off. If all seems plugged in and it still doesn't find your drive, try swapping to the other set of hard drive cables in the tower. Update: Just realized that you are on an iMac. If you hear clicking then it is most likely the hard drive. If you have the discs, does it allow you to boot from them? If so, boot off of the discs. Once booted from the discs launch disk utility and see if your internal drive shows up in the list on the left. If it does you may still be able to recover the data off of the drive by using a spare firewire drive to install the system and boot off of. If you need instructions for this post here and I will try to help.