I sometimes get this kind of problem when I've almost polished a piece (its up to speed, and I'm working on tweaking the little quirks). I get a few bars where I had the notes down pat, and suddenly I can't play it properly anymore. Like most people have said, I just try it slowly, and then I speed it up gradually with a metronome. If that doesn't work, I don't worry about it for a couple days, and it usually manages to fix itself.