My eth0 has gone and I don't have internet and network connection

Try:

sudo dpkg-reconfigure network-manager 

If that doesn't work boot from a live CD, back up your old network settings, clear out any system connection file and copy over the ones from the live CD.

Change to Root:

sudo su

backup:

mv /media/<Name of your Ubuntu Partion>/etc/NetworkManager/NetworkManager.conf /media/<Name of your Ubuntu Partion>/etc/NetworkManager/NetworkManager.conf.broken

clear:

rm /media/<Name of your Ubuntu Partion>/etc/NetworkManager/system-connections/*

copy:

cp /etc/NetworkManager/NetworkManager.conf /media/<Name of your Ubuntu Partion>/etc/NetworkManager/NetworkManager.conf
cp /etc/NetworkManager/system-connections/* /media/<Name of your Ubuntu Partion>/etc/NetworkManager/system-connections/

Can you try to edit /etc/network/interfaces and put it the default settings for DHCP below?

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet dhcp

Try to restart the networking service with:

sudo service network-manager stop

If it doesn't work, then try:

sudo service network-manager stop

Then the following command to bring up the interface:

sudo ifconfig eth0 up

Then, force Ubuntu to ask for a new DHCP lease with:

sudo dhclient eth0

Thanks . It worked for me.

1. I had the IP address but no internet connection
2. The LAN port was active
3. I had assigned static IP address

My /etc/network/interfaces had the following content:

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface

iface enp4s8 inet static
address 192.168.2.251
netmask 255.255.255.0
network 192.168.2.0

I was having ifconfig output as (this is manual edit - it was somewhat like this)

~$ ifconfig
enp4s8    Link encap:Ethernet  HWaddr 00:19:d1:73:b7:11  
          inet addr:192.168.2.251  Bcast:192.168.2.251  Mask:255.255.255.0
          inet6 addr: fe80::219:d1ff:fe73:b711/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:13181 errors:0 dropped:0 overruns:0 frame:0
          TX packets:8462 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:16690503 (16.6 MB)  TX bytes:884958 (884.9 KB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:317 errors:0 dropped:0 overruns:0 frame:0
          TX packets:317 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1 
          RX bytes:40111 (40.1 KB)  TX bytes:40111 (40.1 KB)

I changed the /etc/network/interfaces to:

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto enp4s8
#iface enp4s8 inet dhcp


#iface enp4s8 inet static
#address 192.168.2.251
#netmask 255.255.255.0
#network 192.168.2.0
#broadcast 192.168.2.255
#gateway 192.168.2.1
#dns-nameservers 192.168.2.1

Then I followed the steps included in the answer above:

sudo service network-manager stop
sudo ifconfig enp4s8 up
sudo dhclient enp4s8
sudo service network-manager start

This is when I got the connection to the network, these steps worked.

FYI - The Live CD also had internet connection. I did not copy the files from Live CD.

I've had quite a similar issue after an Ubuntu (12.04) updtate.

I resolved it by logging in recovery mode, then activate the network, then typing into the console:

sudo apt-get remove network-manager
sudo apt-get update
sudo apt-get install network-manager