Every now and then, the above problem gets raised on many Blogger forums. Sometimes the request comes in the form of "My sidebar is missing!, please help". First of all, don't panic, it's not catastrophic.
The most common reason (I would say 90% of the time) for falling down sidebars has been the presence of unclosed <div> tags in one of the blog posts. i.e. a post without one or more matching </div> (note the /) tags for the corresponding <div> tags. Sometimes, it could be some other HTML tag. (For example, I have seen cases where a missing </script> tag caused the sidebar to be pushed down)
If a missing tag is the problem, the following method can be used to identify the post(s) that cause it.
Visit the individual pages of each post that appears on your home page. (For example, if you have configured your blog to show five recent posts on the home page, then visit each one of those five) In one (or more) of those posts, the sidebar will be pushed down, while it will appear normal in other posts. These posts are the ones that cause the problem.
Once you locate the trouble-making post (or posts), open it in Edit HTML mode and check for any unclosed <div> tags. Add the missing </div> tags and publish the post again. That should bring the sidebar back to its usual position.
Though the missing closing tags has been the biggest culprit for fallen sidebars, there are some other, rare reasons such as posting of large pictures or widgets in the sidebar and obscure script errors. If the above method does not fix your problem, then some more investigations are needed to locate the cause.