This seems to be an extremely odd (mis)behavior by CSS Form Builder.
Has this happened on more than one occasion? I assume that the page was OK the first time you inserted CSS Form Builder into the page, but things went wrong when you updated the form? Please could you tell us the exact steps you took to edit/update the form.
When you look at the page in 'Code' view, do you still see the html for the rest of your page, or has the code for your whole page gone too (apart from the form)?
Could you zip the page and attach it to a post here so that we can look at it?