I had a bug in unity once where my project just stopped working. I hadn't changed anything, but I could no longer compile it or run the game in editor. I looked up the issue and apparently unity has/had a long-standing bug where the engine would rarely just quit being able to compile your scripts. The only solution was to make a new project and reimport all your assets, scripts, scenes, etc. Dunno if they ever figured out what was causing it or if it just kinda resolved itself and stopped showing up at some point. I don't really use unity much anymore.