What happens if a payment fails?

Your plan is part of your Shopify bill. So Shopify handles a failed payment the same way as the rest of that bill. If Shopify puts the plan on hold because the bill is unpaid, it starts again once the bill is paid. Your data stays exactly as you left it.

To fix it, pay the open bill in your Shopify admin, or change the card it is paid with. In nouz, open Settings, then Billing, and click Open Shopify billing to reach your bills. nouz never sees the payment itself, so there is nothing to update in nouz and nothing to pay twice.