How can I repair a radiator bleed valve that fell out?
Yes, that is snapped off. You're OK until you need to bleed again. Then you will need to gain enough access to drill a small hole into the broken screw. Once a screw extractor is inserted into the hole, the rest of the screw should come out and you can place a new vent screw in it's place. These screws do not need to be super tight, firmly snugged should do it. At least now you know how much is too much.