This is a rather embarrassing question, but I am rather pressed for time, and can get on with other work while someone far more knowledgeable in Git than me could answer it in seconds.

I've just done a fetch from a remote repo, but I have no working copy files. I'm using Git GUI on Windows, and I can visualise the history, and at some point I could see the file structure of the solution, but I can't find how again. Most operations require me to select a revision, but none are available in the dropdown, and HEAD is not a valid revision for some reason. How do I get the actual source files I need?