Page displaying incorrectly – Stylesheet not working??

I was attempting to fix the drop-down menus on my site and maybe inadvertently did something VERY wrong to make it display like this:

http://www.grkroccenter.org

The only file I changed is style.css, and the only change I made was a z-index number change…

The site SHOULD look similar to this: http://www.kroccda.org

I am a complete WP novice. Please help! Thank you!