Have you got any other NowTV supported devices to see if it accepts your NowTV login details ?
If no forum member on here has any suggestions, then your best bet would be live chat.
Use the green chat button on this link page below to reach the NowTV web chat service.
I am assuming you are using a NowTV box or NowTV Stick ?
And when doing a factory reset on the above devices, you had to setup your wifi network again and was possibly asked to go to the NowTV webpage by entering a six digit code from your television screen ?
If so, did you get past this stage and when going to the NowTV webpage indicated on the television screen your existing account details wasn't accepted ?
Should you have got the NowTV Box or Stick activated again by getting past the stage above, you should have at least access to the third Party Apps on your Box or Stick even if your NowTV account login details are playing up when using the device.
Also double check your bank card details are up to date on the NowTV system.
Anyway live chat would be your best bet, unless another forum member knows what might be going wrong.