Post a screen shot of the Library window so we could see where LR is looking for the photos. I see you're on Windows 10. Are by any chance your photos stored on an external hard drive? If it's the case this could explain what's happening. When connecting an external storage (key, drive; smartphone, etc.), Windows assign a drive letter to it. This letter (D, E, F...) is part of the path to access the files on the storage. The problem is that when you connect the drive, Windows assigns the first available letter in alphbetical order. So, if your drive was connected as "D", the path to access the files on it is D:\<folder>\... But, if the next time you connect it there is a usb key alredy connected, windows having already assign D to the key will assign E to the EHD, and the path to acces its file will become E:\<Folder>\... Since LR records the path in the catalog, this behavior may cause LR to not being able to access the photos. To avoid this problem, I suggest to manually assign a letter far away ("P", for instance) to your EHD. That way, each time you connect your EHD it'll be assigned the letter P, unless all preceding letters are already assigned (which is unlikely!). Of course, after having assign the letter, open LR, right clic on root folder of your photo and select "Reseach the missing folder". Browse to the folder on your EHD and select it. LR will update the path to all the photos under tah folder with the drive letter you've just assign.
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