Would have said an HDMI handshake issue, but you are still seeing the main Xbox menu screen (you could try powering off both your Xbox and TV and disconnect / reconnect the HDMI cable, but i doubt that will work (no harm in trying).
Also have a play around with the HDMI settings in both your Xbox and TV picture /sound menu.
I don't own an Xbox myself, but whenever I get problems with Apps the first thing that I would always try is to delete and reinstall the latest App again.
Here's a guide on how to remove and re install the nowtv App on an Xbox One.
You are in the xbox preview program?
The problem is caused by the xbox preview dashboard.
It effects the alpha and probably the beta rings too.
You should hold the nexus button and select report a problem.
The more people report it to microsoft the more likely it is to get fixed.