Why an application hangs [closed]

I don't use C# but in general, the UI of an application can hang when the UI updating code is not cleanly separated into a thread that is separate from the threads used for network IO (or other non-UI tasks). In such a case, a slow response to a network call can cause the UI to become unresponsive because the network call is blocking the UI thread. This means that UI events (key presses, mouse clicks) are blocked.

IO such as network IO can often be written in blocking or non-blocking styles. Using a non-blocking style might mitigate the effects of not having UI-updating code on it's own thread.

So a combination of a poorly written application and a slow network or misconfigured client (e.g. incorrect address for server) could cause this.

I would say that the UI of a well-written application should not hang unless the CPU is at 100% utilization.

However, this is speculation. The chances of any guess being correct are small. For better answers you may need to provide more details about the application (name, author if retail software) and exactly when and how the UI "hangs".