When Unity won't let you in

Sometimes Unity refuses to open, and that's okay... not?

Before reimporting your project (sometimes this doesn't even fix the problem) or reverting your VCS changes, or applying any suggested fixes: check Editor.log under:

C:\Users\USER_NAME\AppData\Local\Unity\Editor

This will help confirming the issue.

Then, here's a few things you can do:

• Try just opening another project, and maybe re-setting the layout?
• Delete your Packages/manifest.json (backup the manifest so you won't lose track of your custom packages, if any!)
• Delete the /Temp directory next to /Assets
• Delete the editor layout (see below); sometimes Unity crashes on opening custom asset windows (or just any window trying to load broken data)
• Read a long thread!

About the editor layout - this is typically saved in several locations:

• Library\CurrentLayout.dwlt

But also LastLayout.dwlt, which moves around a bit but seen in these places (search within Editor-5.x!)

Windows

• %APPDATA%\Unity\Editor-5.x\Layouts\
• %APPDATA%\Unity\Editor-5.x\Preferences\Layouts\default\

MacOS

• ~/Library/Preferences/Unity/Editor-5.x/Layouts/LastLayout.dwlt

Note: literally pasting %APPDATA%\Some\Thing in explorer does work and faster than figuring where that is!