I have been having a 500 Internal Service Error for days, trying to figure out what the problem was. Spoke with GoDaddy (they tend to have a lot of issues hosting WordPress) and their fixes didn’t fix anything. I eventually disabled all my plugins, and the error went away. I started re-enabling them one by one, and of course the very last one (which was Lightbox Plus Colorbox) caused the error again. I disabled it, and problem was gone. Not sure exactly why it causes the issue, but perhaps your plugin is the same issue. I suggest uploading a fresh version of the plugin before your edits since it was working before that.
