If after several attempts of the thumbnail rebuild you're still unable to view the thumbnails try the following:
Using iPhoto Library Manager to Rebuild Your iPhoto Library
- Download iPhoto Library Manager and launch.
- Click on the Add Library button,
navigate to your Home/Pictures folder and select your iPhoto Library folder.
- Now that the library is listed in the left hand pane of iPLM, click on your library and go to the Library ➙ Rebuild Library menu option
- In the next window name the new library and select the location you want it to be placed.
- Click on the Create button.
Note: This creates a new library based on the LIbraryData.xml file in the library and will recover Events, Albums, keywords, titles and comments. However, books, calendars, cards and slideshows will be lost.
Note: your current library will be left untouched for further attempts at a fix if so desired.