- Quit Photos, if it is running.
- Make sure, your mac is connected to Wi-Fi.
- Then try to drag the photos library from iCloud Drive back to your Pictures folder.
Then wait, until the copy is complete. It may take a long time.
After being on iCloud Drive, your library may need repairing. The iCloud syncing may have broken internal links. But if you moved the library to iCloud Drive, while running macOS 15.3 and not an older system version, iCloud will have recognized the library as incompatible with iCloud and kept it local, without upload to iCloud. Then the library should still work.
Once the library is back in Pictures, double click it to open it in Photos.