Could be a number of things:

1) Board warped in new case or something got damaged in the change over.

2) A "short" between the new case and motherboard somewhere.

3) Is that one of the Cooler Master cases that has a "touch" power button? I have seen start up issues with these cases in the past (they use an ATX power cable that plugs inline with the ATX cable from your PSU).

Other than that, hard to diagnose from a distance. If nothing you've tried helps, take the board out of the case and see if it works normally again. Then check for damage (including socket pins) and check for any potential shorts against the new case and the motherboard.