Scratch

Usually it means it is unable to "save" or "load" data. Like I give you a piece of paper, but you don't get it for some reason. I/O stands for Input/Output ( Input, get something like the Ask Block asks for input, Output gives something like say "Hello" is output ).

Have you tried reloading the page?

Also if a project never fully loads ( just stays as a loading screen ) here might help.