It’s not WordPress that messed it up, it’s something with your theme.
On the working site, your navigation bar looks like this:
<ul class="sf-menu sf-js-enabled sf-shadow">
<li class="page_item page-item-4 current_page_item"><a href="http://www.millermusicllc.com/" title="Home">Home</a></li>
<li class="page_item page-item-7"><a href="http://www.millermusicllc.com/services/" title="Services">Services</a></li>
<li class="page_item page-item-11"><a href="http://www.millermusicllc.com/portfolio/" title="Portfolio">Portfolio</a></li>
<li class="page_item page-item-13"><a href="http://www.millermusicllc.com/about/" title="About">About</a></li>
<li class="page_item page-item-126"><a href="http://www.millermusicllc.com/testimonials/" title="Testimonials">Testimonials</a></li>
<li class="page_item page-item-15"><a href="http://www.millermusicllc.com/contact/" title="Contact">Contact</a></li>
</ul>
On the broken site, the classes on your list are messed up. It’s only rendering <ul class="sf-menu"> and this is what’s causing the problem, because your stylesheet is applying layout, color, size, and other style information based on the missing classes.
So take a look at what you have creating the menu and what code you might have there manipulating the menu. Something isn’t adding those classes to your menu as they should be.
Alternatively…
Your stylesheet, around line 239, has the following declaration:
#top .topmenu ul.sf-menu.sf-js-enabled.sf-shadow li a{/*margin-left:3px;*/font-size:24px;font-weight:normal;text-decoration:none;margin: 0 0 0 -2px;padding: 0 10px;}
This CSS selector is looking for a <ul> element that has all three classes (“sf-menu, “sf-js-enabled”, and “sf-shadow”). If you can’t edit the menu directly, just change this line to:
#top .topmenu ul.sf-menu li a{/*margin-left:3px;*/font-size:24px;font-weight:normal;text-decoration:none;margin: 0 0 0 -2px;padding: 0 10px;}
I tested this through a debugging tool and it does display the menu correctly. But if you were specifically intending to restrict the styling based on the presence of those two CSS classes, then you’ll need to dig a bit deeper.