"the library could not be opened"

Hi

Do you have a backup of the library (eg do you run a regular time machine backup)

Where is the library stored? If it is on an external drive, what is the format?

Try restarting in safe mode.

https://support.apple.com/en-gb/guide/mac-help/mh21245/mac

Safe mode will clear temporary files and caches, and sometimes fixes odd glitches.

(It will take longer to start in safe mode than a normal start)

I've had a similar issue, over the past 2 years, with "Archive" libraries that I've created and stored on an external hard drive.

What I've discovered and confirmed this weekend is that the current version of Photos requires that the library reside on an APFS volume. My external drives (flash, HDD or SD) are usually formatted as ExFAT, for compatibility with ChromeOS, Windows and my cameras

If I copy the library to my local hard drive (AFPS formatted), it opens. If I copy (a smaller test library) to an external drive partitioned/formatted as APFS, it opens.

I haven't found any documentation stating this, but this appears to be the case, so far.