If the pin's gone and the brakes aren't locked, that means:
1. There is no connection to the battery. You should be able to measure 12V on the lead that supposedly goes to the battery.
2. The switch itself is broken. Easy enough to test now that you have a new switch on the way.
3. The brakes are no good. Have you towed this trailer? Are you sure the brakes are working?
4. There is no connection to the brakes. Have you traced the wire running to the brake harness, and verified that it is connected, and not broken?