Do you mean the tab "All Photos" in the main toolbar of the Photos window, when you select your Library in the sidebar?
If all photos are gone after a system upgrade, Photos may have opened a new, empty library instead of your previous Photos Library. You can check, which library Photos has opened by looking at the Photos > Preferences > General: Library location.
If Photos is now using a different library, your previous library may be in an unsupported location. Photos on Sequoia will refuse to open a Photos Library stored on iCloud Drive (for example, if the Library is in the Documents folder or on the Desktop, when these folders are syncing with iCloud Drive). Also, Photos will not be able to open your Photos Library, if it is on an external drive formatted exFAT, or any other file system format that is not MAcOS Extended (Journaled) or APFS. If that should be the case, move your Photos Library to a supported location, for example your Pictures folder.
See the requirements described here: Move your Photos library to save space on your Mac - Apple Support