If the car was in an accident and the aircon doesn't work anymore, it means the gas loop is leaking. You can try to refill it but depending on the size of the leak it's going to work for a few hours to maybe a couple of days. You should evacuate the loop and do a vacuum test. If it is leaking, refilling the system with some added dye can show you where it is leaking. The Schrader valves are the usual suspects but as the car has been in an accident it could be anywhere. Adding refrigerant to a leaking system is just blowing away money that could be used to actually fix the aircon properly.
If you can’t find a sticker (or if that sticker says R-12, it still may have been converted), unscrew the cap on the service port and match it up to the type of port used by each refrigerant.
If it came off then I'd suggest calling a dealer parts department with your VIN and they should be able to get the information.