Photo qui n'apparaissent plus

You have three little circles on some of the thumbnails, which indicates corrupted Library previews.

To fix it, delete the preview cache.

Close LrC. With the Finder, go to the folder that contains your catalog. If you don't know where it is, go to Edit > Catalog settings (might be in the Lightroom menu on a Mac). In the General tab, click Show in the top right corner.

Delete the folder (might display as a file or a package) whose name ends with Previews.lrdata. Do not delete anything else.

The previews will rebuild as you browse folders in Library.

You can rebuild all the previews by clicking on All Photographs in the Catalog panel. Make sure no filters are active, then select all Photos. (Cmd + A)

Then go to Library > Previews > Build standard size previews. If you have a large number of Photos, this can take several hours , so you might want to let it run overnight.

I can't see your file attachment (or anybody's file attachment). Please include your screen capture in your reply by clicking on the "Insert Photos" icon.

From your description of a gray background, perhaps this is the solution: https://www.lightroomqueen.com/how-do-i-change-my-monitor-profile-to-check-whether-its-corrupted/