De link
I had the same problem in F20 with Asus X550C, which uses the asus_nb_wmi module. The hardware switch is a bit flaky, as the led isn't lit up even if wifi is active. Anyway, solution for me is as follows:
- As root, create a file named e.g. asus-wifi.conf and put it in in /etc/modprobe.d/
- Insert this line in it: options asus_nb_wmi wapf=1
- Reboot and see if wifi works.
Works for me just fine, although it doesn't fix the wifi switching with Fn+F2, but that's not a problem for me. More info and where I got the fix from: http://ubuntuforums.org/showthread.php?t=2181558
Possibly just blacklisting the module can work too, as some people reported, but I haven't tried.