I have had similar problems with wlan1 deactivating on a MK5 running firmware 1.0.4. The only way to get wlan1 back is with another reset and re-flash. When gone, it doesn't show in the GUI or CLI commands. Trying to reset it results in a message like "PHY for wifi device radio1 not found".
My "/etc/config/wireless" is the default from the re-flash.
When I run "wifi detect", as suggested above, I get a blank response. Does that make any sense?
root@Pineapple:~# wifi detect
root@Pineapple:~#
Any help is greatly appreciated.