UBUNTU: Network Interface Down On Start Up

I wonder if my fluxbox do cause me with this problem, but as far as I know, this problem occur after a few weeks of using fluxbox as my default wm. At first I wasnt sure if the problem will keep on going everytime I start my laptop, but after a while I notice that this problem will always happen.

So this is the scenario, when I start my laptop> login to fluxbox>my eth0(LAN wired card) will be ilde and cannot do auto IP by itself which other laptop using ubuntu will do.. I wonder what is the problem..

Solution that I found right now is:

• If you are using DHCP for IP then,
• open up terminal, and type "sudo dhclient "
  
• in my case, my eth0 isnt functioning, so my command is "sudo dhclient eth0"
• it will try to fetch IP from the DHCP server on your router or else.
• If you are using Stactic IP then:
• You need to restart your network by using this command "sudo /etc/init.d/network restart".
• hope that it will set your IP to static with out any problem.
• but somehow if this didnt work, so you really need to reconfigure your network-manager. You can reconfigure you network manager by using this command: "sudo dpkg-reconfigure network-manager"
  

I will try to find out why this problem happens as I didnt notice when did this problem starting to cause me trouble. I rarely use my wired cause I'm stuck with wireless connection. My wireless doesnt seem to have this problem. :P