We have C5210 controller with 3605 Ent. APs. THe controller is showing the APs down, with no channel and no power level. However, the switch the APs plug into is up. And, they didn't failover to the backup controller. Their Admin mode says on, but can't ping AP.
It sounds like the AP's have lost communication to the Controller, or network and are possibly still up and active, just not showing the configuration.
I would recommend investigating the ports the AP's are on, see if they are rebooting or staying active, still with a DHCP address, and if they have a route back to the Controller, or if the Controller has a route to the AP's. Can the Controller ping the AP's gateway, etc.
I would suggest opening a case with GTAC if you need assistance with that.