It is due to httpd.conf wrong configuration for your domain. Just do the following:
httpd.conf file and edit that.
Find the "domain.com"
Now, find:
Code:
ServerAlias "domain.com"
And Check whether the following line is just at the end of that para or not:
Code:
ServerName "www.domain.com"
If you are using linux the apache conf will be here:
Code:
/usr/local/apache/conf/httpd.conf
Regards