Quick summary

The catalog file has moved, or the external drive with the catalog is disconnected.

How to fix this

1. If the catalog is on an external drive, reconnect it.
2. If the catalog has moved, browse to its new location.
3. If you can't find the catalog, open it in Lightroom Classic, and go to:
   • Mac: Lightroom Classic › Catalog Settings › General › Show
   • Windows: Edit › Catalog Settings › General › Show
   The catalog is usually in Pictures/Lightroom.
4. Search your computer for files ending in .lrcat.