In Yosemite, suddenly the application Photos is not open anymore Photo.app does not open when clicked on in the dock or in the list of apps.
Message comes up "The application "Photos.app" is not open anymore". How can I re-open it?
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.