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

Question

I had internet connection, but when I upgraded my VGA and restarted my PC then everything for eth0 has gone.

When I used ifdown:

 ifdown: interface eth0 not configured

Is there something I could do?

EDIT:
Content of /etc/network/interfaces

auto lo
iface lo inet loopback

When I ifconfig I don't have these lines:

inet addr:192.168.1.5  Bcast:192.168.1.255  Mask:255.255.255.0
inet6 addr: fe80::219:5bff:fe5e:a5e/64 Scope:Link

In the Terminal typed:

sudo gedit /etc/network/interfaces

and then added these lines in the bottom:

auto eth0
iface eth0 inet dhcp

then I typed:

sudo ifdown eth0
sudo ifup eth0

when I send the second command I received the below message:

No DHCPOFFERS received
No working leases in persistent database - sleeping

Any help?

Answer 1

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/

Answer 2

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

Answer 3

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.