You've got a bigger problem. It should be impossible for that to come off the regulator. You may have to replace he whole regulator. (A component that holds the spring, probably broke.)
Regardless, you'll need to drill out the regulator. (You can only work with the spring with the regulator and motor off the car.) You put a bolt, washer, and nut through the holes in the regulator arm to keep it from chopping off body parts with the motor removed.
When it goes back on, you'll also need an industrial sized riveter and order the special steel mandrel, aluminum jacket rivets needed from GM. (Bolts or plain steel rivets will tear apart the door.)
Black 1999 Firebird Formula WS6; M6
Please DO NOT send PMs for questions that can be answered in the forums!