Please refer Recover catalog and images after resetting preferences in Adobe Photoshop Lightroom and try locating your catalog.
Follow the steps below:-
While starting Lightroom, hold down the Ctrl key (Win) or the Option key (Mac).
This will present a dialog allowing you to choose the catalog you want to open.
Select your catalog and click Open.
1 person found this helpful
Looks like Lightroom is not able to locate your catalog.
Select File > Open Catalog. Then move to the external drive and select the catalog file. It will have lrcat extension.
Hi Akash and Sunil,
thank you, my problem is solved. I took the catalog on my Mac Disk because I thought, the catalog remains with the program files. Now ich found and selected a catalog at the external disk and everything is fine.