OS 10.5 - Ethernet problems

Before, i just started my mac and there was no ethernet connection: I have a cable tester, and the cable is ok.....even the network socket in the wall is ok....
I go to networkng and i see my network interface down (red dot) even im sure the cabe is properly connected and that it should work

I need you to give me as many ideas as you can so tomorrow, when i am again in front of my mac, i can try them to somehow fix the ethernet connection that (i dont know how) is broken now.
Im sorry i cannot give you more details for now.

Thanks for any help you can provide me.

BTW: Linux user, so dont hesitate to put any command i can try.

Thanks!!