replacing plastic heat control cap on trv for myson 2 way?

I had to take the white plastic heat control cap off my myson trv 2 way to fix a cold radiator. But now I don't hoW to Replace it. It just doesn't seem to fit back on easily as if there is an obstruction. When I did replace it earlier by really pushing it down my radiator went back to cold. I don't know on what position of the dial I replace it back to. And why did the radiator go cold again after I had fixed the trv pin?
Really would appreciate any help. Thank you!