Marcel You can see the status of all your cards by going to the "Me" tab and clicking on the "Cards" tile. Greyed out cards are frozen, active cards have their usual color (rainbow or metallic white). By clicking on a specific card and going to the action menu (with the lightning bolt / flash icon) you can always freeze and unfreeze a card.
Make sure you're not accidentally blocking a card, as that action is permanent and cannot be reversed. If you block a card, you will not be able to use it again. So it's better to only freeze a card if you want to make it unusable only temporarily.
