Videos stopped playing from iPhoto

I'm new here, and generally not very good with tech stuff, so be gentle with me!
I've had Mac OS X Lion on my MacBookPro for awhile, and my only issues were with my printer not responding when I print something. Usually I can overcome this with rebooting both. Today however, I suddenly can't play videos from my iPhoto. I went ahead and upgraded iPhoto to '11 thinking that would help. Nope. I get a message saying "a necessary data reference could not be resolved." If I go to Quicktime and open up the files, out of 80 movies, only 8 will play. What to do??

Where did these 80 movies come from? Is there anything special about the eight that will play (different source, different format, something)?

Since you are having other issues, my first recommendation is a course of rehabilitation: run OnyX or Lion Cache Cleaner (or other *forum-recommended* utility) for cleanup, that may make a difference. Make sure you have a lot of free space available on your drive, and recent backups.