You might have damaged the logic board, or power supply in the impact, these are the most likely components to be damaged in a fall thet can result in the machine not booting. if you are willing to open your iMac, you should try and inspect these parts for visable damage. also, an impact like that, almost certianly killed the hard drive, so even though it shouldn't stop the machine from booting, it will most likely need to be replaced.