Website shows Error 500 - Internal Server Error
This could be easily fixed by just changing the permissions of the file/directory in question. You can change the permissions by using the File Manager within cPanel, or an FTP client (for example Filezilla). The correct permissions are 755.
Change all your folder and file permissions to 755 and this should resolve the issue. If this does not resolve the issue delete your .htaccess and refresh your browser.
Other possible causes can be incorrect configuration settings in PHP applications. Check the error logs for a full error and stack trace.