Front End is not working?

I keep getting this message when I try to use the front end? I just bought the High End Theme and I have not been able to use the builder. How do I fix this and does this happen often?

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.