I'm not sure where this post really belongs, so bear with me. I'm putting it here, because it may effect a lot of people, and a warning might not be a bad thing.
Anyway, I was doing a quest and decided to cause a very large series of explosions as part of completing it. After the explosions, I was unable to use my keyboard favorites, or toggle between 1st and third person. I could toggle sneak, jump, toggle always run, but I couldn't use M to see my map, or J to see the data folder.
When I tried reloading a save to get functionality back, I found that reloading the save files changed nothing. I finally found a save from about an hour and a half earlier (which by the way had not been a quick save) that loaded with everything working.
I don't really know what caused this, but I think it's a good precaution to create a manual save frequently...