Synchronizing folders in Library

[Jan5E60]

Problem: my folders in 'Library' are dark grey with an interrogation mark on the right side, and I can't synchronize them any longer. Don't know what happened and how I can undo it (they should be light grey without interrogation mark).

Thanks for a helping hand

Jan 

[Jim Wilde]

The question marks means that Lightroom thinks your folders and images are missing. There are several reasons why that might occur, have a look at this blog post to see if you can sort things out: https://www.lightroomqueen.com/lightroom-photos-missing-fix/