You should check the Apache Documentation on Name-Based Virtual Hosts Support so you might read the following:

Main host goes away

If you are adding virtual hosts to an existing web server, you must also create a block for the existing host. The ServerName and DocumentRoot included in this virtual host should be the same as the global ServerName and DocumentRoot. List this virtual host first in the configuration file so that it will act as the default host.

Edit:

Hence the main Site is online again we should assume on Apaches side everything works. It seems we are not dealing with an issue regarding HTTP/1.1 VHosts (Name-Based Virtual Hosts) since the same features are being used as well for the main site as for the blog.

Now let's see why blog.example.com isn't working. You should check the DNS Record of blog.example.com since it should point to the same address as example.com

$ nslookup
> example.com
[...]
> blog.example.com
[...]
> quit

Edit 2

Regarding what that nslookup did not return a result for the subdomain the problem is explained. The request for blog.example.com never reaches your apache webserver because the DNS server doesn't know what to do with blog.example.com.

So regarding what we see and know, your apache configuration seems to be correct. The main page is online again after we added its VirtualHost. So since I trust you did it right for blog. too.

What follows no depends on the DNS Provider of the domain. The easiest thing would be to put a wildcard subdomain on example.com to redirect to your webserver. This will redirect any request for ANYTHING.example.com to your webserver which handles the rest with the VirtualHosts you configured in apache.

Remember DNS Server take some time before their caches are updated. So these changes might take some time.

The sites-available method is generally considered the "Debian Way":

• "main" config in /etc/apache2/apache2.conf
• "user" config in /etc/apache2/httpd.conf
• vhosts in /etc/apache2/sites-available files (one per file, typically)
  • you might want to number them, e.g. 00-domain.com, 01-otherdomain.com
• ports (Listen directives) in /etc/apache2/ports.conf
• mods in /etc/apache2/mods-available

You can manipulate these with symlinks or with the a2 series of commands:

a2ensite/a2dissite <site_config_filename>
a2enmod/a2dismod <module_name>

Depending on personal preference, you can restart Apache using apachectl, /etc/init.d/apache2 (start|stop|reload|restart), or service apache2 (start|stop|reload|restart)

An example where you would use httpd.conf instead of a vhost entry would be for a global redirect or rewrite rule, for example. Other tidbits -- generally, you should leave apache2.conf alone, and make sure you set up a consistent naming scheme for vhosts in the sites-available directory.