Weird things with the UI locking up or not responding to commands has been an issue for a while. I have had it a few times over the years.
Fortunately, the fix is just to log out and back in, annoying, and not ideal I know.
Sometimes it is caused by a dialogue box somewhere (For instance, the box that pops up saying you have new hardpoints fitted you need to assign a fire group to), but mostly, its just a glitch.