What device are you using to access the NOW TV service?
Have you tried re-booting your router by disconnecting the power, waiting 30 - 60 seconds before reconnecting the power?
Hi again Lou
So you are an Apple Smart TV user, well I'm not and, as such, cannot advise you on anything Apple related.
However, I am sure there are Now TV and Apple TV users that will be willing to offer you the advice and instructions you need.
Still, there is one Apple thing I am aware of, and that is a lot of Apple TV users are having various difficulties and, I believe, are waiting for the new Now TV Apple app to be released.
In the meantime, check out the Apple TV problem pages, there maybe something there you can use:-
When you go into the NowTV App on your Apple TV box and scroll across to settings does it show you are signed in or out of the NowTV App ?
Also, have you got any other NowTV playback devices that you use where you can double check your username and try on the Apple TV box ?
When you initially setup the NowTV App on the Apple TV box did you use an Apple ID or create a Now TV username account ?
If you haven't got any other NowTV playback devices to check your sign in details, then i would check any emails received in the past from both NowTV and ITunes that may give you a clue on your correct log in details to sign back into your ATV box.