LR Classic updated a few days ago and since then, when I start up LR it cannot find my catalog - I have to do that manually. The problem seems to be that it now insists on looking in my C drive.
It seems like every software vendor feels free to use up as much space on the C: drive as it wants, my C drive just fills up and nothing will run. Consequently, whenever I can, I install software on a different drive and configure it to use that drive for storage. So LR and other Adobe software sits on my S: drive and in the past that seems to work fine, but now LR seems to revert to its old habits, looking for my catalog on my C: drive.
Using your system browser, open the folder where your catalog is located and double-click on the catalog. Lightroom should open and should remember where the catalog is located from that point forward. Generally speaking, it's probably a good idea (in my opinion) to leave Lightroom installed on the C Drive. The program itself doesn't take up a lot of space. The catalog and images can be stored on other drives, but you could be asking for problems down the road by having the program on a different drive. The choice is yours, however.
Even better Set the actual PATH, Drive letter, Folder name, LrC Catalog Name, of the catalog you want LrC to open when it starts.
It is on the same page as the option to Open "Most Recent". The LrC Preferences, General Tab.
Click the V on the right and then the Other option. A File manager window will open and then Browse to the Drive, Folder and Your Working LrC catalog file name. then click the Choose button.
That will Set the Actual Path to the catalog you want LrC to use. Even if you open another catalog from either inside LrC or with your file manager the next time you open LrC it will always open the catalog specified in the preferences.