Take off your upper receiver and look in the rear window of the mechbox. If the piston is back you probably just locked it up and the battery is too weak to turn it over.
It sucks that the grip is a part of the lower receiver, otherwise you could probably just do the paperclip trick. In this case, that would require you to pull apart the whole gun, except for the gearbox. If you feel comfortable doing that, let me know and I'll give some more info.