That just happened to me, and seems to be a really common problem! If you live near an Apple store you should take it in there. Are you still under warranty? They'll fix it for you if it is, since it's sort of a design flaw - it happens when the two ridges above the screen hit the palmrest area when you close your computer. However, I've heard of some people still managing to get Apple to fix it for them even when they're out of their warranty, although you might need to find a sympathetic tech in order to manage that. Good luck! 🙂