Are you using Lightroom Classic? (You've posted in the Lightroom forum, which is for the cloud-based Lightroom program).
Your catalog file is a file with the suffix ".lrcat" and that is what you're looking for. Lightroom can open any .lrcat file, but can only open one at a time. However, by using the "Imprt from another Catalog" command, this will allow you to merge the contents of one catalog with another.
If you aren't sure which catalog you want to import, check the "File > Open Recent" items to see the names of other catalogs you may have recently used. Also, you can search the folders on your hard drive for ".lrcat" which will uncover any older files whose location you may have forgotten.