Lightroom CC should have prompted you to upgrade the catalog that you had been using. This would have created a new copy of that catalog in the format required by Lightroom CC, and your work could just continue. If Lightroom didn't prompt you to convert the catalog then you could go to File->Open and choose the catalogs that you have been using previously. Then Lightroom will prompt you to make the conversion. I don't know how many catalogs you have been using, but this process will essentially double the number of catalog files on your system. The new updated catalog will have a -2 appended to the end of it. Then you can continue using multiple catalogs like you have been doing in the past. Or, if you prefer, you can import from the other catalogs and combine everything into one. In either case, it will be necessary for you to open the catalogs and let Lightroom convert them.