I have no idea how this has happened. My site's home page and the next level of pages beneath Home are displaying the main navigation differently than other pages.

Examples...

This is the home page:
https://www.rcu.msstate.edu/

This is the Professional Learning page (a sibling of the Home page):
https://www.rcu.msstate.edu/ProfessionalLearning.aspx

This is the Upcoming PL page (a child of the PL page):
https://www.rcu.msstate.edu/Professional-Learning/Upcoming-Professional-Learning.aspx

The main nav should appear the way it does on the Upcoming PL page.
All these pages use the same skin. Well, the Home page uses a different skin but that skin includes the exact same header file, which is where the nav is.

If the skins are the same, the only thing that can be different is the css.

On the Professional Learning Page, the roxMega ul is styled by roxMega.css.

On the Upcoming Professional Learning page you are using header-lightRCU.css.

I didn't look any deeper than that, but I'd bet that there are differences ...

Today I come back and now ALL the pages' navs are being displayed the wrong way. So strange. How does weird stuff like this happen so much in DNN? I have never seen anything like it. At least now I can fix them.

CSS files change only if someone changes them ...

I doubt that there is something strange or mysterious happening. Check the timestamps on the files and you'll at least know when the changes were made.

It turned out an old version of a CSS file was hanging out in the root site folder. But I'm not sure how it ever got linked to the skin. I deleted the file and everything's back to normal.

And it's odd that yesterday some pages were showing the right nav, and some the wrong nav. I had cleared the cache and checked multiple browsers.

If the file was named portal.css and in the portal root, it would be added automatically.

I have not idea about the pages. Did they go away?