The wallet has two warning states, and they don't mean the same thing. Low balance is a heads-up; blocked is an actual restriction. Both show up as a banner at the top of Billing & Wallet, not buried in a settings page.
Low balance
Under two days of runway at the current burn rate triggers this — nothing is restricted yet, it's purely a warning to top up before it becomes one.

Blocked
A balance at or below zero blocks new deploys outright — the backend rejects them with an HTTP 402 (Payment Required). Critically, this only stops new deploys: services already running keep running, they aren't stopped for an empty wallet.

Warning. Running services keep running, which also means they keep accruing usage charges even while the wallet shows zero — that's how a wallet can end up with a negative effective balance (covered as "pending usage" in "How the Prepaid Wallet works") once a top-up brings it current again.
"Add Credit" (or Auto Top-Up, set up in advance) is the only way out of either state — there's no grace period or manual override beyond adding money to the balance.