sk@ubuntu:~# sudo ifconfig wlan0 up
SIOCSIFFLAGS: Operation not possible due to RF-kill

If you do a rfkill list all you should get an output similar to this:

sk@ubuntu:~$ sudo rfkill list all
0: hp-wifi: Wireless LAN
Soft blocked: yes
Hard blocked: no
1: phy0: Wireless LAN
Soft blocked: yes
Hard blocked: no

Depending on the state of your WiFi button the Hard blocked will either be yes or no. Press the WiFi button and run rfkill list all again to make sure the value for Hard blocked changes.

The actual problem is, that the Soft blocked value is always set to yes, because for some reason the syncing between the hardware block and the software block doesn‘t work as supposed. To override this behavior, you can just run rfkill unblock wifi and it should work again. Double check by entering rfkill list all again and make sure it looks like this, with all values set to no:

sk@ubuntu:~$ sudo rfkill list all
0: hp-wifi: Wireless LAN
Soft blocked: no
Hard blocked: no
1: phy0: Wireless LAN
Soft blocked: no
Hard blocked: no

If you then run ifconfig wlan0 up (or enable the wireless via your desktop‘s network manager) you should be able to connect to a wireless network again without any further issues.

The configuration, which I‘ve been using:

• Compaq Presario CQ60
• Atheros AR5001 Wireless Network Adapter
• Ubuntu 10.10

I also read, that some people could fix it, by turning off the WiFi button during the boot process and then switch it on again, once the system is up and running. It seems that the system expects the hard block to be set to off. This is still a bug, but it could explain why then the soft block didn‘t sync with the hard block any more.

《FROM:http://blog.csdn.net/cat_lover/article/details/7088873》

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UBUNTU14.04也出现过同样的问题，按如上操作后重新启动电脑才能连接到WIFI。

操作步骤是这个的，首先按笔记本电脑的快捷键或开关关闭无线网，然后执行sudo rfkill list all---->ifconfig wlan0 up--->重启电脑。