I just had the dreaded phone call from a photographer friend: "My Lightroom Catalog is corrupted and I cannot open my main or backup catalogs". This is a thing that I can't fix over the phone - It's time for Adobe Help.
They are skilled photographers that do amazing work, but did not know about Saving Metadata to File. This step saves all the work done in Lightroom (except Collections, Virtual Copies, and Flags) into the actual photo file - DNG, RAW Sidecar, TIFF or JPEG.
Then when the Catalog is lost or corrupted, one can open a new Catalog, re-import the photo files, and all the work will be loaded along with the photo: Keywords, Rankings, Color ratings, ITPC, and the photo Develop Edits. You will lose the Develop History, and I'm not sure about SlideShows and Web
Pages, but in a pinch one can live without them.
There are two ways to do this:
1) Catalog Setting to Automatically Save Metadata - see photo.
2) Select the file(s) in LR and Save. Command A (Select All) & Command S (Save) on a Mac. Control A & Control S on a PC.
This is the most common unknown that comes up everytime I do a Lightroom presentation. It's almost a guarantee that 1/3 or more of the group will not know about this.
Note: Collections, Virtual Copies, and Flags are not saved as Metadata, if one loses a Catalog one also loses these - a good reason not to rely only on these tools to manage their Photo Inventory!