Why does my wireless not work now I have upgraded to Snow Leopard?

Hi there

I upgraded my iMac to Snow Leopard yesterday and cannot connect wirelessly anymore.

In my Network preferences both my Airport and Ethernet have got green lights on next to them, but when I unplug the ethernet cable I lose connection to the internet and the green light turns red next to the Ethernet.

I am not very technically-minded and most of the info in my Network preferences is double-dutch to me. I'm sure there is an easy fix for this but I just have absolutely no idea what I am looking for. Please help.

Thanks