Photo library disappeared on iMac

Just butting in here, but I'm really suspicious of that drive-- you can't create a new Library, there? Seems odd.

I think that you should create a Library on your internal drive, add some pictures, and mess around with it to make sure it works. Then, copy that to your external drive and see what happens with it there.

If you can't create a working Library on your internal drive, then it would seem to be your user account. So trying an alternate user would help narrow it down. If a working Library won't function on your external drive, then that's something else.

By the way-- have your re-started the Mac? Have you tried re-starting in Safe Mode?

           Start up your Mac in safe mode - Apple Support 

Hold down the option key and launch Photos. It will offer a menu that should show where your library is located:

Do you have a back up?

As a test: create a new user account (or just use another one if you have it) and see if you can create a library there. If you can we'll know the issue is something in your User account, if you can't it's something in your OS.

And this is with a new library? Is this library on an external drive? If so, what format is the drive it needs to be formatted Mac OS X Extended, Journaled or apfs.

And with that amount of information, nor do we.

Have you looked in your Pictures Folder?

What happens when you launch Photos?

I absolutely would be making a back up of the original files immediately.

As for recovering the files to a LIbrary

https://www.youtube.com/watch?v=5IvSPjs0GSM. Check out around the 5 minute mark.