How the room could be opened again
So Joe through the key in the room. However, the room hasn't been "reset". You can see this cause the final shot is of the key laying on the floor, not reset in its original position.
Essentially the room doesn't know that no one is in it. It might think someone is there.
However with no one to will/think of a door to exit onto, the room will exit onto a random location.
So somewhere in the world is a random door that will open into the motel room with the keys still waiting on the floor. Some unsuspecting person will be very confused.
I think this is how the show was maybe meant to continue next season.
Also for kicks. My theory on what the EVENT was: this was 1961 in the middle of the dessert. Definitely either some sort of experimentation on relativity that split the universe or something, or some kind of aliens Roswell new Mexico thing. It's no coincidence that this happened in the middle of nowhere.
Maybe no one will ever read this, lol. This show is pretty old now. Just finished it and that thought occurred to me.