I've heard these things just die after awhile. It's a 'cheap' brush-type motor so the life expectancy isn't anywhere near that of a non-toy brushless motor. Amazon and eBay have replacement parts. One vendor claims to have 'upgrade' motors, but I can't vouch for them fitting.
Edit:
Have you tried connecting one of the other motors to that channel? If it runs then you know the original is dead. Otherwise it sounds like the board if the leads are soldiered well.