IPhoto lost my pictures, where can I find them?

Question

I usually upload all of my photos straight into iPhoto.
But now, all of a sudden, when I turn on iPhoto, all of the picture folder icons turn on with their key photo,
and almost half of the folders disapear instantly. I can not find those pictures anywhere on iPhoto.
When I turn on email or pages, and want to insert a photo from iPhoto into an email or document, they do show up.
Where on iPhoto can I search for those hidden photos?

Answer 1

Option 1
Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose to Repair Database. If that doesn't help, then try again, this time using Rebuild Database.
If that fails:
Option 2
Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one. 
Regards
TD