My Chrystal Ball seems to be out of order right now, so more info might help. Have you tried to fix it? Is Airport turned on on your Air? Is the network in question still running? Is the AP Ext powered on? Are you within range of the network? Can you connect to or even see other networks?
The Airport network gone. I can't see anymore at the network list.
What OS ?
What model or how old is the wireless router/modem ?
Have you tried rebooting said router/ modem ?
Need more info and does your MBA connect with other wireless networks ?
Is your Chrystal Ball broken too?