system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening when a computer program is not responding?
gpt
It could be a number of things, but generally there's two main culprits:

The first is that the program is behaving fine, but it's waiting on something. Could be data from a hard drive or network, or CPU time. Computers are only as fast as their slowest component. A bottleneck can prevent a program from making progress on its task, which leaves the program in a waiting state. If a program is waiting for too long, the operating system might assume there's something wrong with the *program*, and will mark it as not responding.

The other possibility is that there is something wrong with the program. It might have run into an error that's it hasn't been programmed to be able to recover from or respond to gracefully. Something like an infinite loop or a missing piece of required data can trigger this. If that happens and the program doesn't immediately exit, it will kind of just sit there like an idiot until the operating system notices and sorts it out.