I am having an issue with my site displaying the "Internal Error 500" message. I change the index file name, yet achieved nothing. I changed the content file name however it come up saying that the theme isn't there in a completely different dialogue box. This happened after I used GoDaddy to update my plugins... Regrettable mistake as I hadn't even backed the site up for over a month now.
Basically the idea is to rename the plugins to disable them. Assuming from what you said the plugins are the cause of the issue. To be sure you can always check the error logs on your hosting control panel.