|1. For starters, motherboard voltage is DC, not AC, so it's very possible that shorting out the underside of the board did no damage.|
2. As I've said a couple of times now, proper CPU & heatsink installation are critical. If it is done incorrectly, the system either will not boot at all or it will immediately shut down. That means zero or minimal voltage flowed thru the board. Basically what I'm saying is your mistake may have saved the board from damage.
3. Your power supply is a low end piece of crap. You're putting your whole system at risk by using it, so another possibility is the overloaded power supply sent a power surge thru the board, frying the board, or CPU, or RAM, or video cards, or any combination.
You need to tear the entire system apart & bench test it with minimal hardware as I listed in response 8. Until you do that, we're all just wasting our time guessing what might be wrong.