How many shared albums and how many photos do you have in the shared albums? If you are having many shared albums, it may take several days until they will come back.
As the shared albums are a free service, we are expected to be considerate and not to use it excessively. Apple will limit the network traffic for the shared albums, if we are exceeding the limits for shared albums - then we have to wait until the limits are no longer exceeded. My Photo Stream and Shared Albums limits – Apple Support
As the name says, the shared albums are meant to be used for sharing photos, not as a long time storage of your Photos Library. The shared photos are optimized for sharing - reduced to a smaller size and many metadata removed. We need to keep the originals in our Photos Library to keep them in a good quality and with all metadata like the locations.
If the Shared albums are really missing and it is not a problem of the limits, you need to use the backups you made, while the albums have still been there. How long have the shared albums been missing?