Howdy punyawee,
I understand that you're no longer able to turn your iPad Air's Wi-Fi on. In such a situation, if you haven't already, resetting the devices network settings can help restore Wi-Fi functionality.
Check if the issue still exists after each step:
- Make sure you have the latest version of iOS.
- Tap Settings General > Reset > Reset Network Settings. This will reset all network settings, including Bluetooth pairing records, Wi-Fi passwords, VPN, and APN settings.
If you still need help, contact Apple for support and service options.
If Wi-Fi is grayed out or dim on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT204324
Regards!