Welcome to the Apple Support Communities. If you are having issues with Photo Stream on your Apple TV, The following article gives an explanation and a resolution for the issue.
Apple TV: Photo Stream unavailable after updating Apple TV software
Another option is to sign out and then sign back in to the iCloud account again.
On Apple TV, after signing in with your iCloud account, you can view your photos by choosing Photo Stream from the main menu (or selecting Internet > Photo Stream in Apple TV Software Update 5.0).