Some of those Ares G36's came with upgraded springs for importation 430fps range. If you still have the stock spring in there its an easy fix. Chances are the gearbox is locked with the piston back.
You need to either press the bolt release located in the trigger area and get it to unlock, or remove the spring at the rear of your gearbox via the quick change option and gently press the piston forward with a pencil while holding the bolt release.
Put a lighter spring back in and you should be good to go.
"Remember...stop, drop and roll...will not work in hell"