First thing I would do is make sure the AP600 is still joined to the WLC. This will require you to log into the WLC and make sure you see it in the wireless tab. I would check the logs to see if it flapping. That would be the first step.
We have a large deployment of Ap600. We find the solution very reliable and easy to set up and deploy. I cant say we had one where the SSID just stops working.
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HTH
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