My four-year-old MacBook Air won’t turn on. It’s plugged in but the light is not on the charger. I know the outlet is live because I just used it for something else. What is the sequence for pressing several keys simultaneously?

Try resetting the SMC http://support.apple.com/kb/ht3964

1 Choose Apple menu > Shut Down.

2 After your Mac shuts down, press Shift-Control-Option on the left side of the built-in keyboard, then press the power button at the same time. Hold these keys and the power button for 10 seconds.

3 If you have a MacBook Pro with Touch ID, the Touch ID button is also the power button.

4 Release all keys.

5 Press the power button again to turn on your Mac.

Try resetting NVRAM/PRAM http://support.apple.com/kb/ht1379

1. Shut down the computer.

2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.

3. Press and hold the Command-Option-P-R keys.

4. Turn on the computer.

5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.

6. Release the keys.