How to solve the problem of Lightroom reporting a folder as missing
If a folder is missing, it will have a question mark beside its name in the Lightroom library.
You can sort out Lightroom’s problem by locating the missing folder – to do this, right click the folder name in Lightroom and choose Find Missing Folder.
Now navigate on your disk to find the folder that Lightroom can’t find. When you have done this all the contents from that folder will be added back into the Lightroom catalog automatically.
The reason that Lightroom can’t find your folder is because you have done something to it outside Lightroom. In future, move and rename folders from inside Lightroom and this problem won’t occur.