Okay so I have lost 50lbs and ive been working out everyday and eating healthy?

but today i had a hamburger from carls jr the one with double meat and cheese and some fries with BUTTER MILK and some diet coke i havent eaten this in like 6 months, will i gain the weight back? and will it ruin my diet? i will start working out and to my normal routine the next day.. please serious answers.