TRV problem?

Hi
I turned the TRV fully off while I was painting the room. Now I'm finished, the valve only opens two thirds of the way and wont go to max. I checked the illustrations on Screwfix and it looks like a Danfoss RAS-C2. Its fitted horizontally and has a black collar nearest the rad which slackens when turned. I'm a bit apprehensive about fully unscrewing it as I havnt attempted a repair before. Any help would be appreciated.