Lightroom - File not found -

Hello, 

I have had this issue a few times now and it is rather distressing.

Somehow I've changed something in the catalogue by accident, and during the process it would not enable me to cancel or turn back.

My photos are both still in Lightroom and on my desktop so the 'file not found' is a little confusing and is preventing me from making progress with my work.

How can I restore this?

Thanks