Uhoh, I broke it!

Yikes! I am working on building my first WPMU community. It's been flowing very smoothly and am excited to get it going. At some point, when adding something today, I broke the site.

I can access the main page fine, but when I try to click on anything in the Menu, I get a 404 error.
Here is the URL:

I have tried switching themes and get the same error.

What would be the most likely problem for this type of problem?

I have added some plugins today, but was mostly checking things in the admin panel and didn't realize the pages weren't working for quite a while.

Thanks in advance.
WPMU newbie!