Are the valves on both sides exactly the same?
Just manual valves?
If it's broken in the open position, or can be opened with water pump pliers, I'd be highly inclined to leave it well alone.
That thin bore pipe coming in from below is a total pig to work with, and will kink for looking at it wrong.
Sometimes, simply unscrewing the nut can twist the pipe into a helter-skelter.
Then, you need to look at how far back you can get to splice a new bit of pipe in.
Is it a floor-up job?
Unless you are very confident in your plumbing skills, no good will come of trying to replace that valve.
If you are *very* lucky, a replacement valve will have the same dimensions and thread, allowing you re-use the olive and nut on the pipe.
Life usually does not work that way.
You can tell if it's open or not by whether the thing gets hot!
Presumably you closed the valves to remove the radiator, so you must know whether it broke before you re-opened them or not?
From the picture, the remaining sticky-out bit is the bit that needs turning, and there's enough protruding to get a tool on it to open it.
You probably do want to open it, as bathroom towel rails are often the 'bypass' to allow flow when all the other thermostatic valves have shut.
Like I said, unless there's a compelling reason to replace it, leave it alone, open it in-situ with tools as required.
