Photos can't find my library

If you moved it and know where it is, double-click on the Library file to open it in Photos. The new location will then be remembered. You can also perform a Spotlight search for the Library file.

Otherwise, launch Photos while holding down the Option key. This should then show all Photos libraries that Photos/Spotlight know about. Choose the one you want.

Please read the following user tip and restate your question/problem in detail: Writing an Effective Apple Support Communities Question - Apple Community