It isn't as if they are gone completely, but there are little blank spaces where 99% of the photos should be (I've attached a photo below). I
What happens, when you click a photo to enlarge it? Does the photo reappear? If yes, try to repair the thumbnails.
Your screenshot is showing the Photos for Mac application, not the iPhoto application. Are you now using Photos for Mac?
Then try the procedure described in this User Tip: How to Rebuild Thumbnails in a Photos Library
Occasionally the thumbnails in the Browser may not correspond to the enlarged image, or the thumbnails may appear to be corrupted, but the enlarged image is fine. While iPhoto and Aperture have a tool to recreate the thumbnails, such a tool is missing in the first Photos version 1.0 to 1.5.
We can use this simple workaround to fix the thumbnails:
To rebuild the thumbnails edit them slightly, so Photos will be forced to render new thumbnails. This is best done by rotating the photos back and forth, because rotating by 90° degrees is non-destructive and can be applied to multiple photos at once without having to open the Edit window for each photo individually.
- Select all photos that need new thumbnails at once - it does not matter if the selection includes photos that do not need repairing.
- Then rotate the selected photos back and forth by first pressing ⌘R (Image > Rotate Counterclockwise) and then back by ⇧⌘R (Image > Rotate Clockwise).
This may not suffice, if your photos are RAW photos and the corrupted thumbnails are caused by a difference in RAW processing. In that case try to reprocess the RAW files:
Double-click a photo in your Photos library to open it, click Edit in the toolbar, then choose Image > Reprocess RAW.
Before applying this fix I'd repair the Photos library, just in case there are other hidden corruptions. If your library is an iCloud Photo Library wait with repairing as the last resort. Repairing will result in uploading the complete library again and comparing the photos to iCloud. You may not want to have that happen on a slow Wi-Fi.To repair the Photos Library:
- Make a backup copy of the iPhoto Library.
- Hold down the key combination alt/options key and command key ⌥⌘ while double clicking the Photos icon. Click the "Repair" button and repair the library. This may take a while for a large library.
Update: There is now an Apple Support Document describing this fix:
Fix missing or incorrect thumbnails in Photos - Apple Support https://support.apple.com/en-us/HT205069
