Yeah, it's kinda unusual for a motor or ESC to just "go bad" with no warning. You'd think something would get hot or a little puff of smoke would warn you. IIRC, there was a similar case like this a while back and I think it turned out to be a motor to ESC connection issue even though everything looked fine.
Good luck. And if all else fails, at least there is the holiday sale for the 9920 going on right now...