The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, admin@localhost 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.
when seeing this message, most likely you just make some changes in server configurations or use htaccess w/o knowing what it does, so you should undo what did and find out what it does before applying