This can be caused by a number of issues:

• A missing 'index.html' , 'index.htm' , 'index.php' file
• A misconfigured , hidden '.htaccess' file within your filespace
• Your website may be stopped in MyPanel
• Capitalisation of your 'index' file  - index files need to be 'small case' i.e. 'index.html' and not 'Index.html'.