Hi everyone.   This is the first time ever this has happened, so I wouldn't know how to replicate it. I very quickly knocked up a file with existing symbols, just snapping them together, many layers, many classes.... Then I hit the undo button and the file went back to a state that was some 15 minutes before!!! Redo didn't do any thing, and all the new classes and layers vanished, back to only 2 or three.   It was a new file and I hadn't saved it yet... Ha