Recent account: the "Payment required" page
On recent accounts, your messages are paid by card through the top-ups of your balance. If a payment fails (expired or declined card, or limit reached), the Payment required page appears and blocks access to the whole app until the invoice is paid.
Follow the instructions on the Payment required page to pay the pending invoice.
If your card is the issue, update it in the billing portal (see Updating your billing and payment information).
As soon as the invoice is paid, access is unlocked immediately.
Older account: billed directly by Meta
If your account is billed directly by Meta, your invoices are paid in Meta's Billing Hub, available from the workspace menu (click your team name, then Settings) > Invoices. As long as a Meta invoice remains unpaid, Meta may stop your messages from being sent.
Avoiding late payments
Check your card's expiry date and replace it before it expires.
Make sure your card accepts payments of the top-up amount you chose (see Managing your billing threshold and Auto-refill).
To find your invoices, see How do I view my Meta invoices?.