Move Photos

Your library has been upgraded to work with Photos on Ventura. Unfortunatly this means Photos 7 on monterey cannot open it.

Do you have a backup from before you upgraded? (Hopefully you do, and can restore this version of the library). If not, is your library already synced with iCloud photos. (If it is, you can sync iCloud to a new empty library)

If you don't have a backup, and you don't have your photos in iCloud then it becomes quite difficult. The only choices you will have will be to upgrade again back to Ventura. Then you can choose to continue using that, or to sync your library with iCloud (you'll need to pay for at least one month of a plan big enough for your whole library), then downgrade again and sync a new empty library with iCloud to downgrade it.

For more info, see:

Downgrading a Photos Library to a Previou… - Apple Community

You should be able to use a Ventura install on external HDD.

However, don't drag images into a folder - if you drag out of photos into a folder you only get a lower quality preview file. Instead select the images you want to export and use file>export or file>export unmodified original.

Then import those back into the Monterey library.

And in the future, create a test library to use with Beta's. Otherwise this will happen all over again. It could even be that a Beta Photos Library is not compatible with the Released Photos Library and then you would really be stuck.