Photos cannot find the System Photo Library

Photos has developed a glitch, having been fine.

The Photos application will not open and gives me the message "Photos cannot find the System Photo Library". I can select the library - it is visible and located on the MacBook, see image attached - but Photos says it is still unable to find it. I can create a new library, but still get the same message - again see image of additional Libraries.

The problem occurred when using OS 10.15.3; I have updated to 10.15.4 but the problem is the same. Photos is version 5.0. The size of Photos Library is 72gB. Libraries 2, 3, 4 and 5 were created as tests and are 0kB. Nothing has changed, except that the machine crashed of its own accord one night recently and needed a restart; not sure why. All other applications and functions seem fine.

Any help gratefully received. Thanks!