Ethernet has a self-assigned IP address and will not be able to connect to the Internet

I was using the internet just fine and then all of a sudden it stopped working - I can use wireless, but when I go to the network system preferences it is yellow and says self-assigned IP. I have changed the cables from the router and I get a green light saying it is working. Have restarted multiple times. No idea why it just stopped working? nobody else in the house is having a problem!