photo library disappeared on MacBook Pro after updating Big Sur

Where did you store the Photos Library? In your Pictures folder or on an external volume?

How did you upgrade to Big Sur? Did you wipe the drive for a clean install or just ran the upgrade?

If you just installed the upgrade without wiping the system volume, the library should still be there where it has been before the upgrade. Photos may be looking in the wrong place. Quit Photos, if it is running, then double click the Photos Library to open it in Photos, if you can see your Photos Library in the Finder.

Photos may not be able to use the Photos Library, if it is on an external volume, that is not compatible with Photos. If the Photos Library is on an external volume, it should be prepared as described here: Move your Photos library to save space on your Mac - Apple Support