but when I click on the miniature I only get a black picture with exclamation mark.
That means, that iPhoto cannot find the original image files.
Make a backup copy of the library and try to rebuild the library, as described by Terence Devlin in this post:
After downloading Yosemite, I can't open my iPhoto!!! Please help me!!!
Terence DevlinMar 30, 2015 8:38 AM Re: After downloading Yosemite, I can't open my iPhoto!!! Please help me!!!
Re: After downloading Yosemite, I can't open my iPhoto!!! Please help me!!!in response to dlpowers16
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.
Regards
TD
If rebuilding does not work, look into the iPhoto Library. Select it in the Finder and ctrl-click it. Select "Show Package Contents".
Check the folder "Masters". Are the original image files inside?
If not - is there a folder "Old Masters"?