Hi! My name is ***** ***** I am an independent computer professional who's been paired with you via JustAnswer.com.
If your MacBook is randomly shutting down despite having a charged battery, etc., it's usually indicative of a problem with the device's logic board. That's not something you can fix on your own, unfortunately...it is a fairly complex hardware repair that will require Apple's (or an authorized repair shop's) assistance.
If your computer is less than a year old -- or you have an extended warranty on it -- the repair should be performed, respectively, for free or for the cost of your deductible. Otherwise, the repair is moderately expensive, costing in the neighborhood of a couple hundred dollars.
Your repair options can be found online at: https://www.apple.com/support/macbookair/
Try this first:
1. Shut down the computer. 2. Plug in the power adapter to a power source, connecting it to the Mac if its not already connected. 3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time. 4. Release all the keys and the power button at the same time. 5. Press the power button to turn on the computer. 6. Press and hold the Command-Option-P-R keys before the gray screen appears. 7. Hold the keys down until the computer restarts again at least twice. 8. Release the keys and immediately after you hear the startup sound, press and hold the Shift key. Release the Shift key when you see the Apple logo appear on the screen. Then login and see if it still shuts off by itself.
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