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

Before using Linux Mint Debiab Edition(LMDE) I was using Ubuntu-12.04LTS .
It was my worst experience with ubuntu. It looks like Ubuntu12.04LTS is not matured. I've faced lots of problems like touchpad freezes , after updates system get crashed , lots of bug reporting etc . Anyway I'm not anti-ubuntu .
In frustration I removed Ubuntu12.04 and installed LMDE on my Dell Vostro-1540 (core-i3).
Since the kernel is built with LMDE 201 109 486, then the default kernel is only able to detect a single core . Result was ,I've experienced some sort have 'hang' during my Android application development .
I decided to install the kernel 686, exactly 686-PAE. This kernel is the best solution for running LMDE on 'multi-core' system .
No need to worry about it .
Simple steps are here to install linux-kernel-i686.

 $ sudo apt-get update
 $ sudo apt-get install linux-image-686 linux-headers-686-pae

for i486.

 $ sudo apt-get update
 $ sudo apt-get install linux-image-486 linux-headers-486

for amd64.

 $ sudo apt-get update
 $ sudo apt-get install linux-image-amd64 linux-headers-amd64