Hi
Where and how did you save the library off the old hard drive? Is it possible you saved it to the pictures folder, and overwrote your original library? In this case you will need to recover it from backup.
Alternatively - is it just that your photos app is now opening that recovered library - so you are not seeing your old photos and not those in your current library. If so:
- Hold down the option key while starting photos.
- The resulting pane will (should) show all the libraries on your system
- Select the library you want to open and click "choose library"