Need more information. A "normal" catalog update should not have missing photos. If all of the photos are still where they have always been, and all external hard drives (if there are any) are still connected and active, and you are in fact using the updated catalog, you should simply be able to continue as normal. That has always been my experience.
You have most likely (accidentally) upgraded the wrong catalog file. Use File->Open Recent and select each catalog shown there until you find the desired catalog showing the desired photos. If that doesn't work, use your operating system's search feature to search all hard disks, for files whose name ends with .LRCAT. Double-click on each one to open it in Lightroom until you find the catalog that shows the desired photos.