Photos app (iPhoto) hangs when trying to open Ive got a mac mini running OSX 10.10.5. I use the built in "Photos" app (formally iPhoto) to manage photos i take on my iPhone, which are synced back to the Photos app via iCloud. 

Recently when i open the Photos app it just hangs on this screen (see attached screenshot). Ive force quit the app and re opened it a few times and the same thing happens. 

What can be done to get it to work ?
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.