This just happened to me a couple of weeks ago.
The nylon gear inside the gearbox that is above the window motor cracked. When the cracked part of the gear passed the worm drive of the motor the gear opened up and jammed in gearbox.
I found I could manually wind up the window with the emergency handle (from the toolkit) an inch then electric wind it about 3" until it jammed again.
I've bought replacement gears (in aluminium) and will be installing these as soon as I have the time.
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