I have tried rebboting my Windows 10 PC but I keep getting the same error. I can open a new catalog, but not the one I always use. And I have made sure no other application is trying to open this catalog.
Please go to the location where the Catalog is saved, and please delete the file .LOCK.
Then go ahead and double click on the .LRCAT to open the Lightroom Catalog.
Retrieving data ...