“Not Responding” appears in the title bar of a running program when Windows detects that the program isn’t behaving properly. Exactly why depends on the specific program and what you were doing at the time.
Let’s look at some of the possibilities which range from actual software or hardware problems, to user impatience.
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Programs are expected to respond to Windows
Windows expects a running program to “interact” with it, and respond to its requests in a timely fashion.
If you type a key and the program you’re using doesn’t take it because it’s too busy doing something else, that’s “not responding”. If you click on the Close Program “X” on a programs’s window, and the program doesn’t acknowledge it, it’s “not responding”.
When Windows asks a program to do something, like take a keystroke or close itself, and the program fails to acknowledge the request within a certain amount of time, the program is classified as “Not Responding”.
If the program never comes out of that state, we might also call it “hung”, as in “hung up” on something.
There are “legitimate” reasons this can happen.
For example, if the program you’re using is doing some very long, CPU-intensive calculation, it might not respond in a timely fashion. An example might be a video editing program performing some operation on a large video, or a spreadsheet performing a lengthy calculation. If, during the operation or calculation, you attempt to close the window and nothing happens, Windows might eventually add “(Not Responding)” to the title bar. It tried to pass your request to the application, but the app’s not listening.
It’s bad design to not respond while performing lengthy calculations, it’s also quite legal and actually legitimate. Once the calculation is complete, the program starts listening and responding again.
A different example might be a program trying to access something over a network like the internet. If the connection is bad, or the remote server is slow the program might spend a long time waiting for a response. During that time it might well become unresponsive. Once the response arrives it becomes responsive once again.
Anything causing an application to stop responding to your input or to Windows own internal requests, can cause “(Not Responding)” in the title bar.
Not-so legitimate causes
While lengthy calculations are one semi-legitimate cause, there are many other possibilities as well.
- Programming Error: The classic case of an “infinite loop” in programming is perhaps the most common example. If a calculation is mistakenly written such that it never ends, and during which Windows is never given a chance to operate, the application may become “hung” and unresponsive. As a user of there’s not much you can do except avoid whatever it is you did that brought the application to this point.
- Software Design Error: Really just a variant of the preceding point, but I think of it as a different class of problem. Sometimes a program displays an error message in a pop-up box. While that error message is displayed the application’s main window will stop responding until you dismiss the message. If the message box is displayed off the screen or behind the application’s main window then it will appear as if the application is unresponsive.
- Hardware: Malfunctioning hardware can, in some cases, cause the software interacting with it to fail in ways making it unresponsive. For example a USB reader or thumb drive might experience a failure such that the next program to attempt to access it — even just to see if it exists — might end up “(Not Responding)”.
- Hardware Drivers: All the hardware on your system is in some way controlled by software. If that software has a bug, even if the hardware is working properly, the result could be a hung application. If hangs appear to be related to interacting with a specific device it might make sense to make sure you have the latest drivers for that device — not to mention the latest updates for Windows as well.
- Malware: Whether intentional or simply because they’re poorly written, malware can cause other programs or Windows itself to become unresponsive. Make sure your security software is running and up to date.
It could be anything
Pretty vague, I know.
It boils down to “it could be anything”, which is unfortunately fairly accurate. The actual cause, and the solution, will depend on the specifics of what you’re seeing.
Does it happen all the time, or only when you do certain things? One specific application, or several? All of these things and possibly more are clues necessary to ferret out the cause and come to a solution.
Without more specifics, my general recommendation is to make sure all your software is up to date, all the drivers are up to date, make sure security software on your system is working and up to date.
If the problems persist, try to narrow down the common causes if any.