I Photo

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.   

Could you please add some background information, so we can better understand what might have happened?

Where are you keeping the iPhoto Library? Is it in your Pictures folder or on an external volume?

What is the last thing you did with iPhoto, while it has still been working?

• Did you move the library somewhere else or restore it from a backup?
• Did you import new photos?
• Did you open the library from a different user account or on a different computer?
• Did you run any cleaning applications to save storage?

Which version of iPhoto and macOS X are you using?

Did you make a back up?

Drag the iPhoto Library from your Pictures Folder to another Disk. This will make a copy on that disk.