Sidebar disappears in Explorer

Thanks. I just ran it through the validator at W3C and there were several problems.

I had used HTML to create some horizontal rules underneath the Headers in the widgets in the sidebar, because that’s what the design calls for. W3C said I should be using CSS to create those instead. But I couldn’t find which document has the CSS for the sidebar widgets?

The Facebook Like Box plugin/widget I’m using also had several validation errors that I don’t even understand. I had used the widget instead of directly pasting the code directly from Facebook because it looked like I would have to paste some of it into the HTML file, but I don’t seem to have access to that (remember that they won’t give me access to the FTP account, so I can’t download the HTML file).
Here are the errors regarding the like box. What do they mean and what can I do about it?
Warning Line 435, Column 382: Element name fb:like-box cannot be represented as XML 1.0.

…r=”006b9b” stream=”true” header=”true”></fb:like-box><div><a href=”http://cmsv…

✉
Error Line 435, Column 382: Element fb:like-box not allowed as child of element aside in this context. (Suppressing further errors from this subtree.)

…r=”006b9b” stream=”true” header=”true”></fb:like-box><div><a href=”http://cmsv…

I moved the Facebook widget to the unused widgets area (because I didn’t want to lose the settings) and now the sidebar is completely empty in Firefox and Chrome on my Mac and Explorer on my PC.
All of the other widgets were working fine on Friday (except on Explorer).

Our IT person pointed out that all the other pages except for the front page had the sidebars.
So that made me realize it must have something to do with the fact that they have had me change the front page a few times and it may have been linking to a different version of the front page.
So I deleted the old version of the front page as a blog post and re-created the Mission statement front page, but that seems to have permanently broken the front page: I get a “Nothing Found” error message.
I had also created a new page to hold the blog called “SCIENCE” and I set that page’s order in the Reading settings page, but it won’t show up in the navigation bar. When I go to All Pages in the dashboard, everything is there, as it should be.
Any ideas how to fix this?

Yes, it is a static front page. I have checked Settings > Reading repeatedly. My front page is set on “Mission” and my posts page is set on “Science”. I even tried temporarily changing those settings and then changing it back to what I want, thinking that would re-set it, but it didn’t work.
I also tried re-typing the URL in the browser, thinking that might re-set it. It seemed to work at first, but then I clicked on each of the links in the navigation bar to test them (they all worked fine), and when I clicked back to the Mission page it was back to that “Nothing Found” error.

It’s Twenty Eleven, it came with WordPress. But I have to use a lot of Plugins to customize it, because the IT person where I work won’t give me FTP access so I can’t make a child theme.