'Internal Server Error' indicates an error was thrown either in your web server or in PHP. There should be a log somewhere on your server that indicates what the exact error was - it could be anything from not having enough free disk space in your quota to a hardware issue on the server, and a thousand other possibilities. If you aren't sure where that error is being logged, your hosting provider should be able to find it for you.
Once you have the exact error message, it may point you to exactly where the problem is. But if it doesn't, post it here and we'll see if we can help you get to the bottom of it!
--Joshua Warren - @JoshuaSWarren on Twitter
CEO,
Creatuity