My gf drives an 88 Pontiac Sunbird and whenever she tries to turn off the car the key gets stuck in "on"..help?

It will work if I turn it to start and push the switch in while I turn it off, but if the car is running, I don't want to turn it to start to be able to do that. Could it just be the tumblers are getting stuck or maybe cause it's -20 degrees outside? Please help