I was working on the Admin of my new site, adding products and categories. When I go to add categories, I get an Error 500 message. So I've found all my logs, but I have no idea what I'm looking for. And when I find this whatever-it-is, what am I supposed to do with it?

(Background - This isn't my first site, and over the past 7 years, I've learned to do some coding work on my own with the help of a couple of friends. However, I'm far from proficient. My web developer was creating this new site for me, but it's like he's dropped off the face of the earth, and my new site was supposed to be up this past Monday.)