Sometimes a program may crash or show deranged behavior and one of the dialogues cannot be accessed. This just happened to me. A program looks like it is stuck in a thread issue. When I look on its thread list it seems to be waiting for user input. There is no dialog visible. Clicking on the program’s window just gives the waiting mouse pointer. Sure you can just kill the program but this may not help you find out what the problem really is.
I remember years ago I used a utility that could show these hidden windows. Good luck searching on web for something like “show hidden windows” or better phrases. You won’t find it. Well, yea, you could. I did. But, I’m good, smirk.
Note that this utility is pretty basic, just shows a list of window objects. Some of them should not be unhidden, they will lock the utility itself or cause other problems. Perhaps there is something out there that is better? Seems this should be part of the Sysinternals utilities.