Disappearing photos.

I have had a recent 'clean-up' by Mackeeper.

Now we know the cause of the problem.

Get rid of the app. Never install any app that "cleans" "tunes up" or "protects" your Mac. They're all a waste of money and cpu cycles.

As for your Library: Restore from a back up.

What that twlls you is that the thumbnails are missing, not the images themselves. Editing forces the creation of new thumbnails.

Quick question: have you used any app to "clean up" your mac recently?

Hitting the space bar enlarges the blank so I click on EDIT and the picture appears. I do a brief EDIT, click DONE and photo is back in album. This is very time consuming if a lot of photos are missing. What is going on? Bring back i-photo,

A quicker way would be to select all blank thumbnails at once, for example by holding down the command key ⌘ key wile tapping one thumbnail after the other. Then rotate the selected thumbnails back and forth using the key combination ⌘R to rotate them and ⌥⌘R to rotate them back. This will save you the trouble to open each photo in EDIT mode.

TD asked about cleaning software, that may have damaged the library. Another common reason for blank thumbnails could be iCloud Photos Library in combination with lack of storage on your Mac. iCloud may simply need to download the originals from iCloud, but cannot, because it is low on storage. Is iCloud Photos Library enabled, and do you still have plenty of free storage on yourMac?

Mackeeper is "junkware" and should be uninstalled.

Malwarebytes is highly recommended by most helpers here. :)