Weird thing was, when you clicked into a post, the sidebar was fine.
What I know about HTML could fit in a sock, and the thing that sock-full of knowledge would be most useful for is beating my computer. But I don't give up easily, so I checked the coding and tweaked and prodded and finally, cursed at my computer and spent twenty minutes throwing beans at it.
Later, I checked the Blogger help pages. I did everything they said to do, and nothing happened. I realised it was probably a Page Break issue, but couldn't fix it even with the HTML fixes on the blogger help pages.
HERE'S HOW I FOUND THE PROBLEM:
I narrowed down the possible problems to three posts and a gadget - the last things I had been working on. The sidebar and gadget HTML looked fine. Only one blog entry was behind a page break. Bingo.
HERE'S THE SOLUTION:
I know it may sound painful and/or tedious, but this is the best way to do it and it will fix the problem in a flash.
1: First, copy everything in the blog post you think is causing problems and paste it into NOTEPAD. Making a copy of the post text will save you if you accidentally delete anything or have to try again. It is important to use Notepad because it is a basic text reader/writer and so it doesn't put in formatting that can screw with your blog post. Microsoft works or other rich text "advanced" word processing programs will cause more problems.
2: Now that your post is saved, press the edit pencil to edit your blog post.
3: Make sure your editor is in the "Compose" section, not "Edit HTML".
4: Select all text and press the "Remove formatting" button. You can find it in the right-hand corner of the editing bar. It looks like a T with a red X beside it.
5: Re-publish your post with the "Publish Post" button.
6: Check Main Page.
Voila! Your sidebar should be back to normal. Unfortunately, your post will have lost all bolding, italics and if it contained pictures, they will be in the wrong places, but that's fixable. Oh, and I was able to put the page break in when I was finished and it didn't hurt my sidebar at all!
Now tell me, wasn't it worth the time?