My project no longer saves in Scratch 3.0. I tried refreshing and going to My Stuff but that did not help.
The project does not contain any large files, but there are large blocks of code.
To try to fix it, I moved the sprites into the backpack to copy to a new project. However one of the sprites failed to copy.
Apparently there was a line of code that got corrupted. Deleting and then creating the line again (no changes) fixed the problem.
The backpack helped find where the problem was. I deleted various parts of the code in the sprite that failed to copy and then tried to copy it again. This process eventually led to the single line of code causing the problem.