Hi and welcome,
Use iPhoto Rebuild Photo Library dialog to restore missing thumbnails.
So possibly try this,
1) If iPhoto is running quit the application.
2) Hold down the Command and Option keys on the keyboard, open iPhoto and keep the keys held down until you are prompted to rebuild the library.
3) Rebuild the photos small thumbnails may already be selected.
4) As you can't see a photos unless it is blown up to full size, check Rebuild all of the photo thumbnails.
5) Click the Rebuild button.
Let us know how things go.