...and remember your towel!
I quite frequently see (not responding) on the Title Bar of Revit these days, more now than in the past it seems. It can be a bit disconcerting however don't panic. It is usually just Revit refusing to talk to Windows while it is cranky through whatever task you've given it. It's as if Revit is saying, "Hang on a minute, I'm busy, I can't be bothered to tell you (Windows) what I'm up to at the moment."
So when you see the message, don't panic, give it a few minutes, check out Task Manager, see how much memory is being used. Make sure the computer is actually "stuck" before doing something drastic and using "End Task".