I was configuring PPPOE connection manually by command line using "sudo pppoeconf", and by mistake I changed some settings. So after I restarted my laptop, I was getting an boot time error "waiting for network configuration" and after some time "waiting upto 60 seconds more for network configuration.."
Also there are possibilities to get this error while upgrading the system.
I solved these problem by using 2 methods.
After logged into Ubuntu.
Open terminal, type
First : not a permanent solution

  sudo /etc/init.d/network-manager start

Second : permanent solution

  sudo gedit /etc/network/interfaces

And remove the lines after

  auto eth0
  iface eth0 inet loopback

Make sure to save the file before closing it.Reboot the system