In trying to repair a pipe I went to empty my baseboard heating system, and so I tried to open what I thought was a bleeder. It didn't seem to make any difference, and now after unbolting it it will not thread back in.
Is this actually a bleeder? Either way, any idea why it won't close?
It's a balancing valve, and you have probably got the guts of it confused with excessive unscrewing. You may be able to sort them out if you undo the BIG nut and take them out. if not, a heating professional can.
The purpose of this type of valve is to balance flow between heating loops, so it will also need to be re-adjusted once reassembled (or replaced.) The lack of a handle is intended to keep it from being fiddled with by the average homeowner, as it is a "set and forget" device if adjusted properly in the first place.
Hint for the future - bleeders don't face down.
