WordPress themes are for sure a good delight for eyes , but they look equally odd when some components change their position even though you did not touched any theme file, well the same case happened with me actually it was the case with one of my blog where on the index page the right sidebar would drop down, while on the single post page it was all right Now that made me go mad ,as i checked through all my CSS and template files and every DIV tag was closed sidebar was included at proper place and so on, but there was no solution the sidebar was still DOWN.
I still cant believe that this problem had a very simple solution , just go to your WordPress Dashboard then to Settings->Writing-> and select the checkbox that says “WordPress should correct invalidly nested XHTML automatically” as shown in the figure below ,
Do this and your problem will be solved Ok, happy now !!
Now, the technical side of this solution is that due to some plugins or auto formatted copy paste content etc. our Post HTML contains somewhere a closing div tag like </div> without a matching opening DIV tag, what this does is that it prematurely closes the content Div and so sidebar falls down.
Also, Sidebar drops sometimes due to extra width of the content ID or class, so check your stylesheet as to if the width of content column +width of sidebar is less than the tootal width of containing column ie. the wrapper or the body selector.
Anyways , hope that helped