The warning triangle means, that iPhoto has lost the connection between the thumbnails and the photos and cannot show them in the browser.
Rebuild the iPhoto library to reconnect the photos. What is your iPhoto version? If it is iPhoto '11 do the following:
- Back up your iPhoto library.
- Hold down the command and option/ alt keys while double-clicking the iPhoto icon to launch iPhoto.
- Keep holding down the keys until you are seeing the FirstAid panel.
- Select "rebuild database" from the panel.
Does that bring your photos back?
If not, download iPhoto Library Manager and use the rebuild function.
See Old Toad's post: