Photos app (iPhoto) hangs when trying to open
The first thing I would try before doing anything else is giving the Photos Library Repair Tool a go.

The Photos Library Repair Tool can resolve a number of issues as it analyses the entire database and, where possible, makes repairs. 

Follow these steps:


Hold down the Option + Command keys as you open the Photos app
The following message will appear:





Click the blue Repair button to repair your Photos Library.