Forget everything you know about it and break it down and re-learn it as though you haven't looked at it before--it's probably a mental block. I have had lots of times when I just couldn't learn an "easy" piece because of the mental block aspect.
This is also a reason I try to convince my students to never learn any piece hands separately--each hand can seem simple alone, but once you try to put it together, it's like learning an entirely different piece that's four times more difficult!
Another bit of great advice I got from a professor long ago--never, ever, choose fingerings while playing with just one hand. One hand can influence the other, so make sure you are using both hands when choosing fingers.