Linux Lost Internet Connection

Linux renew ip command

The -r flag explicitly releases the current lease, and once the lease has been released, the client exits. For example, open terminal and type the command:
$ sudo dhclient -r
Now obtain fresh IP:
$ sudo dhclient
There is no need to restart network service. Above command should work with any Linux distro such as RHEL, Fedora, CentOS, Ubuntu and others.

On a related note you can also try out the following commands:
# ifdown eth0
# ifup eth0
# /etc/init.d/network restart
OR
# /etc/init.d/networking restart 

Source: http://www.cyberciti.biz/faq/howto-linux-renew-dhcp-client-ip-address/

OR

 $ sudo dhclient eth0

Source: http://askubuntu.com/questions/2901/unmanaged-network-icon-network-manangement-disabled