I saved, came back and my work wasn't there

My work wasn't on anything. No files, autosaves or anything. After trying to recover my work, to no avail, I decided to start over. I finished the first two things I had to do in my video, and was ready to start the third. I took my hands off the computer for a second and everything was gone. Undo and redo did nothing. I'll start over later but I have to know what happened and how to prevent it.