Why do I see a folder with a question mark on it when i try to start up my MacBook Air?

Your Mac cannot find a viable version of Mac OS to start up from. Boot into Startup Manager and select your Mac OS volume.

How to select a different startup disk - Apple Support

If that doesn't work then boot into Recovery Mode and reinstall Mac OS:

About macOS Recovery - Apple Support

Click here and follow the instructions. If the computer was running Mac OS X 10.6.8 or earlier and you reach the third set of steps, insert a Mac OS X install disk or plug in its original thumbdrive.

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