Open the Hop app or myhopcard.com and select “Report Lost or Stolen.” That will deactivate the card, but your balance will stay on your account. You can use the Hop app to reactivate your card if your phone turns up, or transfer the card to a new phone.
Or, you can use Apple’s lost-phone feature: Log in to icloud.com and select Find My iPhone, then select your device. “Lost Mode” will temporarily suspend use of a Hop card on the device until it is authenticated.