How do I actually diagnose why software broke instead of just restarting?

You can restart and reinstall, and often that is enough to get things working again. But understanding why it broke feels out of reach, and you sense that guessing is not the same as knowing. That instinct is correct, and the fact that you want the real cause instead of a lucky fix means you are ready to actually learn this. Diagnosing is a habit of mind more than a body of knowledge. It starts with a simple question asked patiently: what changed? Software rarely breaks on its own. Something shifted, an update, a setting, a file. When you slow down and trace backward from the symptom to the change, the mystery usually has a trail. One step: next time something breaks, before you reinstall, write down what the error actually says and what happened right before. Follow that thread as far as you can. You learn diagnosis one traced problem at a time, and each one makes the next clearer.