Thumbnails mixed up / orig photos ok. How to reset linking?

Hello,

I'm on a MacBook, using iPhoto '11, version 9.3.2. I have over 9000 photos in iPhoto.

One recent iPhoto Event with ~500 photos has suddenly developed a problem-- the thumbnails are completely mixed up, while the original photos are fine.

If I am in the Photo Editor mode (? where you double-click on a photo and see it enlarged, by itself), the photos are in the correct sequence. I can command-I and see that the photos still have the correct file names and times.

If I am in the thumbnail viewing mode, the thumbnails are completely shuffled with no rhyme or reason, and certain ones appear multiple times. If I command-I, I see the information of what the photo *should* be (the actual file information), not what the thumbnail is.

The strange thing is that this is only for this one event-- everything else seems to be ok. These photos were taken on a Canon camera, but the camera is a year old and this problem has never happened before with it.

I tried going into my time machine backup, but when I get to the iPhoto Library and try to open it, the "open in iPhoto" button registers a click but does nothing.

In the forums I found a few discussions of incorrectly linked thumbnail problems like this but they were from 7-8 years ago and the solutions weren't working for me.

thank you in advance for anyone who has some advice!