I did the exact same the night before my first weigh in and luckily managed to lose. I know that doesn't always happen for everyone but the only thing I can recommend is dusting yourself off and carrying on. Realistically, you're not going to not have nights out for the rest of your life just because you're following Slimming World, it's how you move on from it that matters. Don't make over indulgence worst by deciding you've blown it and that you'll just have any old thing...stay on plan from here on in, SW is for life, so make it work for you.