When EQ is performing some functions Windows can display "Not Responding" in the titlebar at the top and it can make it look as though EQ has crashed. This does not mean it has crashed. You should not end the program, doing so may cause data corruption.
Why does it show?
EQ is usually very busy working in the background if "Not Responding" is shown. Windows applied the text to EQ when EQ is not refreshing the screen very often, so Windows may assume that EQ will not respond to any future commands. However it usually does!
When do you see it?
You could see this message when EQ is doing very processor intensive tasks such as loading a large catalogue or importing or exporting a large spreadsheet/CSV file.
Factors such as the speed of your computer processor, the number of other programs you have open, the speed of your network can all impact EQ's performance. You should try to wait for the process to complete, EQ will become responsive again once it has finished.