What was it that made you cave? It's not a disaster if you can learn from it, and do better next time. 
Every set back is part of the process, it can be more beneficial long term than a success! 
You're not at square one, you've still consumed fewer calories than you would have of you were starting today. Just take a deep breath, and carry on. 
A set back is only a disaster if you allow it to become one x