You should contact your bank to check why your payment was refused. For privacy reasons, your bank is not allowed to disclose the reason of refusal.
A common reason is that your account was overdrawn at the moment when the payment was requested from your bank. Try topping up using a different payment method available on the website. If everything is back to normal, you can always let us know by email and we will reactivate the text and/or automatic top-up service again.
What can I do when my direct debit is blocked?