Code appearing at top of page.

Strange page code appearing at top of page today. Have not change or done anything to plugins or code. The site has been just siting there for the last month. Called my web hosting and they checked the server and said their end was OK and suggested I disabled my plugins. When I disabled all-in-one-seo the site sort of returned but missing boarders and tab shapes.

WP 3.4.2 Atahualpa 3.6.4 Site http://2bretired.com

Any help on trouble shooting this, the following problems:-

Home- missing boarders and has square tabs
About us/contact us tab - missing borders but has rounded tabs
Latest blog post tab - goes to complete page code, with part of page missing header at bottom.

I am not sure where to start as I have not touched or done anything to the site to cause a problem. The site has been fine up to now. Do I need to upgrade to the latest Atahualpa.
But what is causing the existing problem?

Thanks Bob