If your MacBook Air won't turn on, despite being connected to a power source, and you have already tried resetting the SMC (System Management Controller), here are a few other troubleshooting steps you can try:
1. Check the MagSafe connector: Ensure that the MagSafe connector is plugged in properly and that there is no debris or dirt preventing it from making a proper connection. Also, check the MagSafe cable for any signs of damage, such as frayed wires or exposed metal.
2. Try a different power source: Plug your MacBook Air into a different power source, such as a different power outlet or a different charger, to rule out any issues with the power source itself.
3. Perform a power cycle: Disconnect the MagSafe connector, hold down the power button for 10 seconds, then release the button and reconnect the MagSafe connector. Then, try to power on the MacBook Air again.
4. Check for signs of life: Look for any signs of life when you press the power button, such as the sound of the fans or the light on the keyboard. If you hear or see anything, it could indicate a problem with the display or the internal components of the MacBook Air.
If none of these steps work, you may need to take your MacBook Air to an Apple Store or an authorized repair center for further diagnostics and repairs.