when I pull the triger my gun makes a loud bang and dose not fire

It seems like your O ring might have broken.

The plunger is retracted into place when you $!^& the blaster, and then when you pull the trigger it is released and the spring sends it forward to deliver a blast of air to propel the dart.

If your O ring is bad, than you no longer have the sealing effect on the plunger and it wont be slowed by air pressure being forced out through the cylinder and barrel.