Is there any parental restrictions set on your console?
Is your consoles region set to a region supported by Now Tv?
If both of the above are OK, then it's probably time to clear the System Cache:
When the Xbox is powered on, press and hold the power button the console until it shuts down completely.
If neither of these work, it's probably time to try a factory reset.
Go to Settings>System>Console & Info Updates>Restory Factory Defaults.
Hope these help!