If nothing is physically fried (IE, you don't see anything burnt,) have you taken a look at the soldering? Despite being manufactured en masse and supposedly identical, I've seen blemishes in toy soldering all the time. A bit of jostling in the right way (like, say, opening the thing up and moving the wires about a bit,) could have very well damaged any really crappy soldering.
If that's not the case, I...really don't know what to tell you.
