Refit old style Danfoss TRV?

One radiator was cold ( not often used). I managed to take off the Danfoss cover & tapped the valve a few times. Almost immediately I heard the water flowing & then the pipe & radiator warming up. Success! But I'm having difficulty refitting the cover. I think it snaps back on but not having much success. In the meantime the radiator is permanently on fully unless CH switched off..I don't know the model but could upload photos if that is possible.