OK, here's a weird one.
We have two Apple Airport Extreme routers in our facility. They are at opposite ends of the building and don't see each other. We have them configured for the same WLAN name, but with different channels even though they are far enough apart to not see each other. We have had this config for at least two years without issue.
Today, for no reason that I can see no one can connect to the one in the back of the building. We have throughly checked the config. Thinking that perhaps the device had failed in some manner, we saved off the config and got out a brand new Airport and dumped the config onto it. However, no one can connect to it either.
I then brought the old one up to my area where I have a working good one and I powered the working one down so that there was no chance of interference and I powered up the other one; still can not see it. We have changed every setting we can; WLAN name, turning off Encryption, changing IP and adjusting all of the wireless settings to all their possible values; nothing will fix this issue.