If your library is large, the restoring may take more than a day. Just wait and don't interrupt it. Photos has a lot to do to upgrade your Photos Library to a new format. How large is your library?
While you are waiting you Amy want to check, if your Photos Library is stored in a safe location. Photos 9 on Sonoma cannot work properly with a Photos Library stored on iCloud Drive or on an external drive, if the file system format is not MacPS Extended (Journaled) or APFS. And the drive needs a wired connection, not a network connection. The eternal drive must not have been used for Time Machine backups or have other backup software installed. If you Photos Library is syncing with iCloud Photos, make sure, your Mac is connected to a fast and reliable Wi-Fi. (Move your Photos library to save space on your Mac - Apple Support)
As my Photos Library is syncing with iCloud and all my photos and videos are already in iCloud, I did not wait for Photos to finish the "Restoring". After waiting for a day I created a new, empty Photos Library and set this up to sync with iCloud. Photos downloaded a fresh copy of my library from iCloud and I could work with my photos again after a few hours.