What's a comfort relay? :P
Your cassette may be binding. They can be repaired, depending on what is wrong or course. You can replaced the sliders, but they are a major pain. I wouldn't recommend doing it yourself.
That might be your problem. Maybe you could open it up and inspect, lube, clean the rails and sliders, etc. Then it may not shut.......so.........
There may be a manual handle in your tool kit in the trunk, which should allow you to manually move the cassette, to override the motor. It will either fit into the motor, like a hex bit, or the motor should be removed and the tool used instead...one or the other, if you have a tool at all.