MacBook Air has no power
i have tried to charge my computer but it doesn’t boot up. The screen is black. My power cord light was on the whole time, but never turned green . Can I use a USB cable to charge it?
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No, you cannot use a USB cable to charge the 2017 and earlier MBAirs.
Try an SMC reset. Pay attention to the Magsafe LED as it should change to green and amber for a brief moment.
Try unplugging the power adapter from the electrical outlet for a minute to reset the electronic circuits in the power adapter. Also try using another electrical outlet. See this Apple article to check out the power adapter and the Magsafe port on the laptop.
https://support.apple.com/en-us/HT203207
It is possible the laptop is on, but the display is dark due to a backlight issue (backlight is set low or maybe an actual failure) or perhaps the display is not receiving a video signal. Is the laptop warm? You can try connecting an external display, then close the laptop lid to force the video to go to the external display to see if you see anything on the external display.
You might be able to try a PRAM reset, but this is tricky to do if you do not know the power on state of the laptop.
If the above does not help, then you should have Apple or an Apple Authorize Service Provider examine your laptop. You may have a bad battery or other hardware issue. Make sure to take your power adapter as well.