Don't worry, you can easily lock your card in the Account section of the app. Just tap 'Lock card'.
Once you've found your card, tap 'Unlock card' and you'll be able to use it again.
If you can't find your card, go to the 'Account' screen, tap 'replace' and follow the instructions.
Alternatively, you can give the customer support team a call on 0800 069 8380.