don't edit Twenty Eleven directly - strongly consider to create a child theme: http://codex.wordpress.org/Child_Themes
it is important to have an unedited default theme as a fallback in case of serious theme problems; also, any customisations of the default theme (i.e. twenty eleven) will be overwritten with your next upgrade of your wordpress version.
if the browser window is narrower than 650px the sidebar will go down to below the content = design feature for modern browsers (IE for instance ignores the corresponding styles which you find near the end of style.css, starting with
/* =Responsive Structure )
this appears directly after
</html> if you look at the html code in the bropwser. difficult to tell from which file this is coming from.
this could be from a broken php tag, possibly at the end of index.php(?) or from a broken plugin.
check index.php - if there is nothing in there, de-activate all plugins and see if this remaoves the ?> at the bottom. if it does, re-activate one pluginat a time to find the trouble plugin.
there are solutions to this in the forum; for instance: