Hi, Don.
The release button is part of a combination safety system. I think it comprises a solenoid, some wiring and a cable, and works in conjunction with the brake servo.
My guess is that the cable was wet/damp and froze up - not surprising outside at -15 F. Running the engine and leaving it for a while let enough warmth seep through to free the cable off.
Just a wild guess, but I have been known to be right sometimes.
Ken.