it could be any number of things, but start with the motor connections- quite a common one for those to pop back off when re-assembling the grip, or to install them the wrong way round so the motor is trying to stall itself against the anti reversal latch.
otherwise it's a case of checking the trigger connection isn't bent, and that all the contacts are in place as necessary (how many and where will depend on how the wiring loom on your gun is made)