kellystarr851 wrote:
.... and learned that I had inadvertently clicked on an old Library in Photos.
That is strange - why did you need to click a library to open? Usually, Photos will always open the last used library when you launch Photos and we do not have to select a library, unless the library is on a volume, that Photos cannot find.
Is your Photos Library in a supported location, for example in your Pictures folder? Or are you keeping it on an external volume? If your library is on an external volume, make sure, it is compatible with Photos for Mac, as described here: Move your Photos library to save space on your Mac - Apple Support
And make sure, the external volume is always mounted and available, whether you are working with Photos or not. Your system Photos Library needs always to be available for he background processes, whether you are working with Photos or not.