From Cards you can issue a virtual or a physical debit card for yourself or your team, see its details and activity, and freeze it at any time.
Freeze and unfreeze
Open the card and choose Freeze. A frozen card declines new payments until you choose Unfreeze. Freezing is instant, so it is the first thing to do if a card is lost or you see a payment you do not recognise.
A card that froze by itself
If your business has a receipt rule and a card payment above the threshold has no receipt, the card can be frozen until the receipt is added or waived. The card page says so. Add the receipt and it unfreezes. See Receipts inbox and receipt rules.
Who can issue cards, and how much a card can spend, depends on your role and on your payment rules. Owners and admins set those rules. See Payment limits and approvals.