Did you actually send youself a test email? Or did you just preview?
You always want to send a test and go through it checking everything.
Not doing so is like heading out on a road trip while being only "sort of" sure you didn't leave the oven and the stove on.
I'm very sorry to hear this happened. Has this been occurring frequently since the change in site interface? If you could, would you be able to take a screenshot or send yourself a test send to your inbox before doing a live send next time? This way we can have something to compare it to and if it did happen again we would have something to reconstruct it with.
We would very much like to get to the bottom of this issue and please reach out to us at social_support(at)constantcontact(dot)com if the issue persists.