inside the deck is a moving transport, the transport engages the tape when the tape is played.
When the transport is engaged, the tape is pressed against the head, the pinch roller has got hold of the tape.
So now, the transport is stuck against the tape, and will not release the tape. The pinch roller has gotten hold of the tape, and can't let go.
This happens when the belt, rubber roller, etc wears out, and the transport can't let go of the tape. The transport is stuck, can't move.
However, manually turning (by hand) inside the transport, you may be able to find the pulley that moves the transport away from the heads.
When this pulley is turned, the transport will let go of the tape, and the door will open.