Bill has a good point,

With Spektrum you must follow the right procedure for turning on and off and if you make any changes to the system, it's best to rebind, especially if you do something with the throttle or any surface. If you forget to turn the radio on first, always shut everything off and do it again.

If you do send everything back make sure you send the receiver battery and switch as well. I know I said we need more info and we really do, but based on your statements, you could have had a battery drop a cell which may only show under a load, your switch could have failed, something could have disconnected (you should always tape, heat shrink, etc... connections to extensions or wherever possible)