When activating your Easy Payment fails, you have two options.
Does your first payment fail? In that case, your monthly bundle will expire immediately. From that moment on, you'll only be able to receive calls and texts, or call emergency numbers. Via your account you can draw up a new direct debit mandate, select your monthly bundle, and off you go again. In case there was usage outside of your bundle in the meantime, this will be billed in your next invoice or payment invitation.
Have you done a successful payment before? No need to rush then. :-) In this case you'll receive a reminder so you're able to pay your balance with a bank transfer. Here too, you'll have to set up a new Easy Payment method via your account afterwards.