ubuntu 16.04, wired connection suddenly lost

Question

I'm an absolute novice to Linux/Ubuntu and network, don't even know how to use vi editor yet. Just installed ubuntu 16.04 for about a week and happy learning online. However the wired connection, which has been fine for a week, lost all of a sudden when I turned on my computer today. I tried to find some solution by googling, and noticed that someone here asked the similar question couple of months ago, which turned out just bad wires. So I checked my hardware, changed 3 cable wires, one is brand new came with my new router (Netgear AC1900 R7000). the following is the info I gathered by reading some posts

ifconfig -a:

eth0      Link encap:Ethernet  HWaddr 00:1a:6b:36:35:51  
   UP BROADCAST MULTICAST  MTU:1500  Metric:1
  RX packets:0 errors:0 dropped:0 overruns:0 frame:0
  TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
  collisions:0 txqueuelen:1000 
  RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
  Interrupt:20 Memory:fe000000-fe020000 


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:2390 errors:0 dropped:0 overruns:0 frame:0
  TX packets:2390 errors:0 dropped:0 overruns:0 carrier:0
  collisions:0 txqueuelen:1 
  RX bytes:202319 (202.3 KB)  TX bytes:202319 (202.3 KB)


wlan0     Link encap:Ethernet  HWaddr 00:13:e8:21:fd:c9  
  UP BROADCAST MULTICAST  MTU:1500  Metric:1
  RX packets:4047 errors:0 dropped:0 overruns:0 frame:0
  TX packets:3489 errors:0 dropped:0 overruns:0 carrier:0
  collisions:0 txqueuelen:1000 
  RX bytes:3294308 (3.2 MB)  TX bytes:485753 (485.7 KB)

some ping test:

$ ping 8.8.8.8
connect: Network is unreachable


$ ping 192.168.1.1
connect: Network is unreachable

route -n:

    Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

ip route show returns nothing.

I could get online by wireless, however the results of ping tests are very odd. It keeps running, I have to break it by Ctrl+C

$ ping 192.168.1.1

PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=2.00 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=1.33 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=64 time=1.33 ms
64 bytes from 192.168.1.1: icmp_seq=4 ttl=64 time=1.33 ms
64 bytes from 192.168.1.1: icmp_seq=5 ttl=64 time=1.36 ms
......

64 bytes from 192.168.1.1: icmp_seq=17 ttl=64 time=0.667 ms

35 packets transmitted, 35 received, 0% packet loss, time 34019ms
rtt min/avg/max/mdev = 0.667/1.024/2.008/0.355 ms

same for ping 8.8.8.8, just the last 2 lines different

--- 8.8.8.8 ping statistics ---
16 packets transmitted, 15 received, 6% packet loss, time 15023ms
rtt min/avg/max/mdev = 9.357/11.939/30.631/5.519 ms

route -n and ip route show gave the following result under wireless connection.

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    600    0        0 wlan0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 wlan0
192.168.1.0     0.0.0.0         255.255.255.0   U     600    0        0 wlan0

$ ip route show

default via 192.168.1.1 dev wlan0  proto static  metric 600 

169.254.0.0/16 dev wlan0  scope link  metric 1000 
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.8  metric 600 

I have not installed or removed anything from last night when everything running normal and alright. I don't know why all of a sudden the wired connection lost, even under wireless connection things don't look right, though I could get online - like post this asking-for-help post.

If you need more information, please be very detail and specific, like what should I type in command line, how do I get out of vi or any other easy editor I could temporarily use.

=========================update========================

the output of interfaces and NetworkManager.conf

    cat /etc/network/interfaces
    # interfaces(5) file used by ifup(8) and ifdown(8)
    auto lo
    iface lo inet loopback

    cat /etc/NetworkManager/NetworkManager.conf
    [main]
    plugins=ifupdown,keyfile,ofono
    dns=dnsmasq

    [ifupdown]
    managed=false

---

So my interfaces was looking like

    # interfaces(5) file used by ifup(8) and ifdown(8)
    auto lo
    iface lo inet loopback

Do I delete these 2 lines or just added your above 2 line in-middle like this

    # interfaces(5) file used by ifup(8) and ifdown(8)
    auto lo
    auto eth0
    iface eth0 inet dhcp
    iface lo inet loopback

---

Right now my interfaces file is as below, but still no wired connection after re-boot (which took about 6 mins)

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp

Answer 1

Your /etc/network/interfaces has no entry for your eth0 connection so it can't really start something there.

If you add the following lines it will be started during boot.

auto eth0
iface eth0 inet dhcp

If you want to have a static IP for your client you can also type in a static IP.

auto eth0
iface eth0 inet static
  address 192.168.x.x   
  netmask 255.255.255.0  
  gateway 192.168.x.x
  dns-nameservers 8.8.8.8

Note that you have of course to ajust the IP's to you network.