Start with:
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 early versions of Library Manager it's the File -> Rebuild command. In later versions it's under the Library menu.)
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.
Re images that are in "Photos" or "Last 12 Months"
If the Photos are under 'Last Import' or 'Last N Months' then they are in the Library somewhere, just not where you are expecting them to be. We know this as both Last Import and 'Last N Months' are not places, just listings of elements in the the Library.
The two most common reasons that they can't be found is either an incorrect date on the camera or the Sorting (View Menu -> Sort Photos - or Sort Events, as appropriate) has changed.
Quick route to find them: Right-click on one of the images in Last Import and select 'Show Event'. This will bring you to the image in the Library.
And as for the switching to 'Last Import':
Go to your Pictures Folder and find the iPhoto Library there. Right (or Control-) Click on the icon and select 'Show Package Contents'. A finder window will open with the Library exposed.
Look there for a Folder called 'Auto Import' Remove anything in it.
Make no other changes. Start iPhoto. Does that help?
Regards
TD
