It's hard to tell what the issue is, unless you provide more info.
My first guess, if a page mysteriously disappears from Google - chances are, it has been penalized.
Why - well, there are many reasons why websites get penalized, including unnatural backlinks, content violations, etc.
My second guess - it has moved to a second/third page.
What position did your homepage had while being on the first page? Are you using any tools for monitoring your positions? Webmaster tools can give you an idea of what's going on.
Hope this helps.