Hi Mark,
your best bet is to do a rebuild choosing the rebuild thumbnail options.
Try this:
Before doing a rebuild make sure iPhoto's trash bin is emptied. You can do this by control clicking on the trash bin, or from the menu bar under iPhoto.
Close iPhoto
Duplicate the iPhoto Library folder
Drag the duplicate to the desktop (hopefully you have enough room on your hard drive for the duplicate.)
Launch iPhoto holding down the Command and Option keys until you see the rebuild options screen.
Choose the first three options. If you were missing photos in your library, also choose the last option. Be advised that you will get a roll of orphaned photos and it might be quite huge. Most of them will already be in your iPhoto library, but use caution when deleting anything from this roll until you are sure the photos are still in your library in another place. It might be a good idea to share/export this roll of photos just in case you need to import them again.
Back to the rebuild, depending on the size of your library it might take some time. When it is done, hopefully your photos will be there.
You might see some unexpected results as I did when I did a rebuild. Photos had been moved to wrong dates, video clips had lost their duration info and some were even orphaned from the jpg pointer file. It took me some time to straighten it out again.
This is the reason to make a duplicate before you rebuild, in case you do not like the result. If you don't like the result, close iPhoto, then delete the rebuilt library and drag the duplicate back into the Pictures folder making sure the name is iPhoto Library. Launch iPhoto and it should open the library in the Pictures folder.
iPhoto 4 or later: Rebuilding the iPhoto Library
