None of these answers really explain how to identify the underlying issue when encountering these symptoms.
If you open your Task/Processes manager before you try to launch your Unity project from Unity Hub, you’ll notice that the a new Unity process actually starts before exiting a few seconds later.
So something should be logged within the Unity log file. To identify the actual issue, locate your Editor log file:
e.g. Windows: %AppLocalData%\Unity\Editor\Editor.log
In my case, the log message clearly shows here that the license has expired. For some reason, the licensing page of Unity Hub doesn’t reflect this information, it still showed another 10 days on the license’s validity!
Once I’d fixed the license issue, the problem was fixed.