I was driving in a parking structure at low speed when a loud bang went off inside the cab. The hazard lights went off, there was an acrid electrical smell, and fine dust was coming from the steering wheel area. I stopped the truck and got out to look for a collision, but there was none. When I restarted the truck, the air bag light came on and I noticed that the seatbelts were retracted and no longer functional. I was not injured, but I believe that if this had happened in traffic or on the highway, I and my passenger could have been seriously hurt. The vehicle was towed to the GMC dealership, where they identified an error with the module that sends a signal to the airbags and seatbelts to deploy. Prior to this occurrence, I had no indication that the airbag/seatbelt system was faulty or in need of service.