I know that if your 9 volt battery is dead, the gun won't allow you to turn it on. The light will go on then off. You said you used a new battery, but you may want to try another one just to be sure. Sometimes you can buy new batteries that are already dead. If that doesn't work, then I'd make sure you connected the leads properly, and that they didn't break off again while you reassembled the gun. If that still doesn't work.....check out the back and front of the board and look for burnt spots. You may have shorted something.
Black/Purple Spire F/s. Freshly updated from Virtue.