Reverting to old commit

Newbie question: What's the simplest way to find an old commit and revert?

Say you have a version from yesterday which worked and today's experiments failed, so you want to go back to that working version. Unfortunately, it's not tagged. Instead you need to actually view the code to identify the version you want. I thought maybe gitk might help, but I can't find my way with gitk. I hoped you could browse revisions easier in a GUI and tell gitk or git-gui to revert?

Thanks,

Marcus