If you think there's a chance you might find your card again, then you can pause it - no one will be able to use it as long as it's paused. If you find your card, you can unpause it in the app and resume using it as normal.

If you're sure your card is lost, or if it has been stolen then block it as soon as you can in the application: click on the "Finances" section on the main screen in the application -> select the card that you want to block -> click on "pause card" -> click on "block". Please keep in mind that you will not be able to unblock your card. You need to order a new one in the application.