You've got a few options here.
Block your card temporarily
In the app (fastest)
Maybe you've just misplaced your card. If that's the case, you can block it temporarily - just head into your Coconut app, open the Card drawer on the "Account" tab, and push "Block". You can unblock it the same way.
You can also block your card by calling our emergency line. You can find the number on the back of your card. Obviously that doesn't help in this scenario, so it's here too.
Cancel your card permanently
In the app (still fastest)
If you're sure it's lost in the ether, or some unscrupulous cretin has taken it, then you can cancel it from your app. Just head to the Card drawer on the "Account" tab and then push Manage > Order new card.
You can then order a new card from the app straight away. Depending on your price plan, there may be a replacement fee for the card, check out our plans here. Make sure your address is up to date before ordering a new one.
If you don't have your phone, you can also cancel your card by calling the Coconut emergency line during office hours.