Is there an 'Error message number'?
As well as the information that @SeeMoreDigital is your ISP BT by any chance?
If your ISP is BT, and there is a feature called smart setup on BT routers, which you may have to disable in order for NowTV to work from what I've seen in the past on the forums. The Xbox one in particular doesn't like the smart setup feature.
NowTV app starts with logo then disappears into blue screen. Xbox appears to be working fine. Have tried usual cache / power drain checks with no luck.
Are you getting any kind of error code or does it just go to a blank screen?
Also have you tried uninstalling and reinstalling the app?