I can't walk you through this with a click by click.

But I will share a STUMPER that I encountered a short time ago. The symptom was just like you presented.

What caused it?

The router was set to Europe and the channel was out at channel 13 since no other routers were found out there.

http://en.wikipedia.org/wiki/List_of_WLAN_channels

It was a real stumper.
Bob