Does it perhaps say something like "Can not find photo"? Can you quote the exact word-for-word message?
If it is saying that it can't find the photo, is it possible that the photo was stored on a disk drive that either went to sleep or was (accidentally) disconnected?
What happens if you re-start the computer? Does the problem go away?
Can you show us a screen capture?
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I must admit I have never seen this exact error message before.
Nevertheless, you are showing collections on the left; can you switch to the Folder panel and find the exact folder that this photo is stored into? Is there a question mark icon on that folder? (An example can be seen in Figures 3 and 4 of this document: Adobe Lightroom - Find moved or missing files and folders)