No photos appearing in Photo library

You do have a backup, right? Go into the backup made just prior to this happening and restore the library.

When you create new photo library as per this article Create additional photo libraries in Photos on Mac - Apple Support

then open finder , take cursor on top menu bar > home > pictures and select the newly created photo library and right click over it and move to trash and then empty the trash .The photo library will be empty .

So , you have to set up the photo library once again , once you open photo application you will see this screen shot .

click on open other , another pop up window appears , select photo library ( just tap over it and click on choose ) .

in photo preferences click on use as system photo library .

check the box of iCloud photo library , and download originals to this Mac and connect with power adapter , with high network speed the photos will start syncing .

Launch Photos with the Option key held down and in the next window see if you have more than one library. If you do open each one in order and see if one of them has your photos in it. You may have created a new, empty library somehow and that's what you're seeing.