Look in your Pictures folder (Go/Home/Pictures). Is your Photos library there? If so, double click on it and it should open. If the library is not there, Option-Click on the Photos app icon in the Applications folder, or in the dock, and you should see a pop up menu with a list of your Photos libraries. Click on one to open it. If no libraries are revealed, then you probably inadvertently deleted it or moved it to a weird location that the Photos app can't find.
All else failing, and assuming that you have no backup of your Photos library, create a new Photos library and copy over the photos from your iPad library.
-- Rich