Instructions
Apologize, even if you don't know what you've done. Emphasize that whatever the misunderstanding is, it must have been caused by you. Do this calmly, and emphasize that it must have been a miscommunication.
Continue on as if nothing is wrong. You've already put it behind you and you would be amazed to find that they're still carrying it around. Just be normal. Capricorn can tend to go off half-cocked, as they say, although they'll never admit it to you. Acting as if everything is normal allows them to save face.
Suggest an outing, a cup of coffee someplace or a short road trip to see some sights. Pack a picnic, and even take along some bubbles or something fun and silly, if it's appropriate.