I had this just happen to me. What I did was I went back to the original theme I downloaded and redownloaded the original index.php. I also found that some plugins does not work with Word Press or I have not figured out how to do so yet. I found that I first needed to add some scripts where they need to be in the templates and tried to activate the plugin again. But in order to do that I had to go through FTP and delete the plug-in that caused the error in order to troubleshoot. At least that will help you to isolate what happened and figure it out.
Hope this helps some. It was very trying for me!
