jg m4 help

[TheGunTrucker]

ok. this is for all of you airsmiths out there.my freind has a JG m4 and it wont fire, so i told him to bring it to me and when he did i took the hand guard off and looked around and them at the fuse and it was popped.so we put a spare fuse i had in it and it popped.so we called black jack a local airsoft shop and the gentle man siad it was the piston bc the motor was turning part way.is this correct or could it be something easle and for shop owners it 60 a ok prices to charge to redo thie piston if i have a spare???
thanks for all the help.