Empathy is your friend here, and empathy in UX is critical. The ability to put yourself in the customers shoes should be intrinsic to anyone wanted to practise UX.
With that in mind, think how you would feel if you suddenly lost access to a service on which you, perhaps, rely heavily. Especially if the reason for non-payment is due to personal circumstances and unintentional.
You haven't stated what type of service you provide nor how users normally pay for it, but these are all considerations.
If, for example, someone pays by Direct Debit and the payment isn't immediate, are you happy to reinstate access on the proviso that the mandate has been set-up?
So my advice would be:
- Ensure the message is clear, but polite and non-threatening
- Ensure you provide clear instruction on how the user can regain access to the service
- Ensure you provide information as to how much is outstanding and what the monthly cost would be to ensure continued, uninterrupted access to the service
- Decide whether there will be any delay in clearing the outstanding balance by the customer, whether you will grant instant access to the service
- Potentially allow the customer a promise-to-pay period, where they inform you of a date they will pay and you give them access to that date, which can be good customer service
- Ensure the message is clear about what happens if they don't pay - account deletion after X days etc.
- Be clear about what payment methods you accept
- Provide contact details if the user has a query, the non-payment issue may be an error on your end!
If you take into account all the information above, you should be able to put together a screen that achieves your aim.