I agree with blasterdemon, you may need to adjust your clutch rod, make sure the little ball is still in there too. I some how lost mine during my last rebuild and my clutch wouldn't engage, which also prevented my blaster from going into gear.
It's easy to do. You take the clutch side cover off and in the middle of the clutch basket is a nut. You loosen the nut and turn the rod till the clutch arm lines up with the mark on the case. It's more detailed in the manual.