More than likely, your trigger contact may have fried. Either that or something with the motor.
If there is no MOSFET in the gun, it is not 11.1 lipo ready. It may be tuned well, but trigger contacts will always be a weak point.
The slowing down sounds like something may have gone wrong with the motor.
But without having the gun yourself to check things, you'll have to ask the person working on it a to what really happened.
If you did have it, I would say take off the baseplate of your motor grip, unplug the wires, and pull out the motor. Check the brushes, brush springs and the tabs that the connectors go on. If everything looks symmetrical on each side, and nothing broken, plug it in OUT of the grip, and test. If the motor spins, then it may have been a bad connection to the motor. If it doesn't spin, it is probably somewhere else in the wiring harness, or the trigger contacts.