WCS is telling you the AP isn't connected any more, so it would make sense that you couldnt do anything to the AP (since WCS controllers the WLC's which control connected APs)...
Your best bet is going to be to track down the AP and figure out what it says is happening. If WCS says its not connected, then it shouldn't be on any of your WLCs either, and if its not on the WLCs you're going to need to get access to the AP to figure out why it is "down".
This could be as simple as the AP has no power (or is dead).....
Switchport is disabled (or dead)....
Perhaps that AP can't get an IP address anymore and just lost IP connectivity?
If you know the AP is up and has an IP address, you might be able to SSH to it (or easy to get Console?) to at least see what the AP says is happening.
-Wesley Terry