Head onto Live chat and Now TV staff will be able to help out.
Just make sure you click "chat online" within the green box.
Alternatively send an email to the address in the below picture.
If it was a small sum of either £1 or 10p then this is a bank card checking method system NowTV use when somebody creates a new account and the money is credited straight away.
There is a paragraph about the bank card credit check in this NowTV article linked below.