sounds like motor or gears
the motor should be an easy fix. small screws on the bottom of the handel should allow you to remove the end plate and view the motor. Pull it out and check the gear for damage. Re-seat with the wires at the back end.
If that didnt work, its gears.
http://www.airsoftplayers.com/mechbox/tutorial.asp
doing the mechbox isnt easy, but its not as hard as it sounds. Just dont loose anything and put it all back the way you took it apart.