Completely remove apache2 from OS, Ubuntu especially, if you removed directory /etc/apache2 when you uninstalled apache2. and you installed again there is no directory /etc/apache2 again. so you must do below step:

Find and drop all Apache packages :

APACHE_PKGS=`sudo dpkg --get-selections | grep apache | cut -f 1`
# Make sure things are sane:
echo $APACHE_PKGS
# Example output: apache2 apache2-mpm-prefork apache2-utils apache2.2-common \
# libapache2-mod-php5 libapache2-mod-python libapache2-svn
# Likely if you have a Python application:
# libapache2-mod-python libapache2-mod-python-doc libapache2-mod-wsgi
# Or if you roll with the PHP: 
# libapache2-mod-php5
sudo apt-get remove --purge $APACHE_PKGS

Re-install Apache

# You might want to consider not re-installing the whole list;
# instead dump it out, audit, and install what you need:
sudo apt-get install $APACHE_PKGS

After this, all the configuration files were back, and I had a default Apache install. Whoaaaa Cool😛

Source http://goo.gl/DdpnK