We have R500 APs and they randomly go unmanageable. The ethernet port remains physically UP, but it doesn't pass any traffic. So far, the only solution we got is to reboot it and we don't want to keep doing that.
What do you see in the logs from the AP and the Switch it's connected to? If controller managed, do you see AP events in the controller logs? What version are you running? Is it latest for your platform? (Solo, Unleashed, SZ, ZD, etc)