Why my website work on: http://example.com but on www.example.com not working? [on hold]

Why my website work on: http://example.com but on www.example.com not
working? [on hold]

<VirtualHost *:80>
ServerName www.example.com
ServerAlias example.com
Alias /static/ /home/ubuntu/proj/proj/static/
DocumentRoot /home/ubuntu/proj
WSGIScriptAlias / /home/ubuntu/proj/proj/index.wsgi
ErrorLog /var/log/apache2/error.log
LogLevel warn
CustomLog /var/log/apache2/access.log combined
</VirtualHost>
Why my website work on:
http://example.com but on www.example.com not working?
EDIT:
If I run www.example.com in browser I have: It works! This is the default
web page for this server. The web server software is running but no
content has been added, yet. From http://example.com work fine.