my nb's airport keeps telling me it has a self assigned IP won't connect

My computer went to sleep as per usual. upon wake up, I had a box that said my clock was set before Jan. 2008 and to change it. I went to change it and saw that actually, in the system perference pane for clock etc. the computer had the correct date and time. then I couldn't connect my laptop to the wifi. I ran the diagnostics and each time, I cannot finish because I get "unable to establish a network connection" it says to go back and change my network connections but I haven't changed them! I went to the gear box and selected positon and moved the airport to the top of the list an have kept trying to run assist me or diagnostics and keep getting that the network conneciton failed and the aiport has a self assigned IP or cannot find the IP. What is the problem(s) and how do I fix it?

thank you.

I have a macbook

Mac OS X Lion 10.7.5 (11G63)

airport extreme