You might have missed a closing
</div> some where, check your errors and fix them accordingly.. Or you could download a Fixed version of this that I have.. takes care of some errors and such.. Also, you might have some images too wide which sometimes, pushes the sidebar down.
But the way it appears...is you're missing a
</div> or something maybe.. My Demo of it fixed is here:
Just unzip it and upload over existing ones..all except the header image..
P.S. By the way, your sidebar.php file is all messed up maybe too, just upload those fixed files of mine and stuff..should be ok..