[RPi] SIOCSIFFLAGS: Operation not possible due to RF-kill

今天要啟用 WiFi 時，出現錯誤訊息

pi@raspberrypi:~ $ sudo wpa_supplicant -i wlan0 -B -Dwext -c /etc/wpa_supplicant/wpa_supplicant.conf 
Successfully initialized wpa_supplicant
rfkill: WLAN soft blocked
Could not set interface wlan0 flags (UP): Operation not possible due to RF-kill
ioctl[SIOCSIWPMKSA]: Input/output error
ioctl[SIOCSIWENCODEEXT]: Invalid argument
ioctl[SIOCSIWENCODEEXT]: Invalid argument
ioctl[SIOCSIWPMKSA]: Input/output error

pi@raspberrypi:~ $ sudo rfkill list all
0: phy0: Wireless LAN
 Soft blocked: yes
 Hard blocked: no
1: hci0: Bluetooth
 Soft blocked: no
 Hard blocked: no

pi@raspberrypi:~ $ sudo rfkill unblock 0 

不知道什麼原因(不是國家沒選)，突然 wlan0 被 block 了。

pi@raspberrypi:~ $ cat /etc/os-release 
PRETTY_NAME="Raspbian GNU/Linux 10 (buster)"
NAME="Raspbian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"

pi@raspberrypi:~ $ uname -a
Linux raspberrypi 4.19.75-v7+ #1270 SMP Tue Sep 24 18:45:11 BST 2019 armv7l GNU/Linux