In Yosemite, suddenly the application Photos is not open anymore
I'd try clearing the saved state files.

From Finder:  


 Cmd ⌘   N   
 Cmd ⌘   Shift ⇧     G      
Copy/paste ~/Library/Containers/com.apple.Photos/Data/Library/Saved Application State


Find the folder com.apple.Photos.savedState and delete it.
That should give Photos a fresh start.