ORIGINAL: rlipsett
it could be the transmitter. One of the big clues is that after changing the expo the servo started to work again. The encoder section of the transmitter might have problems. Send the system back and have it tested out it is cheaper then losing another plane if the problem is with the transmitter
This is a possibility. Either a switch or potentiometer could have become intermittant and no signal was being sent to the TX encoding section or it didn't recognize a change of condition.
Again, box up the whole thing along with a thorough explaination of what happened and what you observed.