What should I try first if my MacBook Air won't turn on after an update?

Try a forced restart first. Press and hold the power button for about 10 seconds until the screen goes black, release, wait a couple seconds, and press it again to turn it back on. If that doesn’t work, try resetting the SMC on Intel-based Macs, or resetting NVRAM by holding Option + Command + P + R for 20 seconds after pressing power. Also plug the MacBook Air charger in — a drained battery can look like a black screen.