Updated Lightroom Classic and lost my catalog

Use your operating system’s search feature to find all files in all folders on all disk(s) whose name ends with .LRCAT 

One of those .LRCAT files found (probably the largest one) is the catalog you want, double-click on it to open it in LrC (and LrC will probably want to upgrade the catalog, which you should agree to).

When you update Lightroom Classic, it sometimes loses track of its catalog. If that happens, it will create a new empty catalog. That is why you think you lost all your photos, but they will still be where they were. Look in your Pictures folder for catalog files (extension .lrcat). If you think you found the correct file, then double click on it. Lightroom Classic will launch, open the catalog and from now on it knows again where that catalog is.