iPhoto won't open

I've had no problem with iPhoto since I installed it many months ago, but now all of a sudden it won't load at all. The program starts to open, but it won't go past just the frame of the window. When I right click it says "Application not responding," and I have to force quit. I've tried repeatedly over the past two weeks and it's always exactly the same. I back up regularly with Time Machine, but I've never used it to retrieve anything yet.

Thanks for any help someone can provide!

Christine