is it a standard gun or has it had the v2/3 conversion?
it could be the anti reverse spring in the gear box, that may have broken or it may have become disconnected in some way
to be honest its a pretty easy strip down job if it is, there are some video's on you-tube.
I am/was in a similar place a while ago with my g&g L85, i bought a none fire'er, which turned out to be trigger contacts, but i did a re-shim/grease job on my gear box, with new piston/piston head etc, sweet as a nut and silent as hell now just fitted the latest flash hider, and hopefully getting DD rails for xmas to bring mine up to the latest spec (minus the acog, as i prefare the current scope i have on mine, see the L85 thread)
edit:- i should also add that mine had a madbull barrel which was causing no end of issues(when i got it running), such as cracked bb's etc, so i got hold of a new G&G barrel and hop