It sucks. You've been screwed over and you can't remember where you last had your card.
If you know you've left your card at home and you've already left for work, or you're sure you left it in that jacket pocket - press Temporarily Pause Card, and if it gets into the wrong hands they wont be able to use it. Embarrassing for them, ideal for you.
You can always unpause it if you find it again!
If your card is no longer in your possession, even if it returns to you at a later date - you'll want to press Report Card as Lost or Stolen. Remember they can still write down your card number and details to use at a later date.
This will block your card and automatically prompt a new card to be sent out to you (make sure your address is up to date!).
Now relax and wait for your new card to be delivered.