my mac book air has a question mark at the start up screen

If your Mac starts up to a question mark:

A folder with a flashing question mark means that your startup disk is no longer available or doesn't contain a working Mac operating system.

If the question mark appears only momentarily before your Mac starts up normally:

1. Reset NVRAM.
2. Make sure that your preferred startup disk is selected in Startup Disk preferences.

If the question mark persists, follow these steps:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Turn on your Mac and immediately press and hold Command (⌘) and R to start up from macOS Recovery.
3. While in macOS Recovery, use Disk Utility to repair your startup disk.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.
5. If you still need help, please contact Apple Support. 

BHP116 Said:

"What type of support do I need for a question mark at start-up?"

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Two Things...

I. Go here: If your Mac Starts Up to a Question Mark - Apple Support

II. Mount the Drive:

Boot into Recovery Mode (Hold Down: [Command + R] upon boot). Then, using Disk Utility, see if the Hard Drive is seen. If it is, then see if you need to click the “Mount’ button for it. Try rebooting once more. If of no success, then click unmount button, and then the mount button. Try rebooting once more.