Hiya!

Can't tell you why you fell off, unfortunately.
What I can tell you is NO, you didn't screw everything up! I fell off on Monday (not my fault really, the restaurant I was at misled me) and literally felt horrible all of tuesday (and put on almost a kilogram from monday to tuesday).
You need to just get back on again. Accept that you're human, we all have failings! No-one's perfect.
It took me 3 days to get back to my Monday weight, but I'm still losing!
You do need to try very hard not to "cheat" but it's not the end of the world if you do! No-one's giving you marks here, the only person who'll suffer is you. You need to decide how important the weight loss and health factors are to you, and keep those in mind next time you're tempted. Take it a day at a time... Don't dwell on two weeks - make your mission each day to make it to TOMORROW!

Hope this helps a bit!
Steven