Nothing dramatic happens on the first failure, and you are told straight away.
The floating Needs your attention card shows a red row: Subscription renewal failed — please update your payment method, with an Update card button that opens the Payment methods tab of Subscriptions & Billing. On the Subscription Plan page, a red panel titled Renewal Payment Failed offers two buttons: Update Payment Method and Retry Payment.
To fix it, press Update Payment Method. It opens the Payment methods tab of Subscriptions & Billing, where you add a working card or make another saved card the default. Then go back to Subscription Plan and press Retry Payment. When it works, a green Subscription Renewed Successfully panel shows your new end date.
If your subscription has already ended, your store is closed to your customers until you renew. The attention row reads Subscription expired and tells you the date to pay before to keep access. Its button says Pay now, or Subscribe when no card is saved. Pay before that date to reopen your store and keep using your dashboard.