MAC Book Air

TBH, this probably isn't the best forum for this question but let's see what we can do.

Can you provide some more details about the MacBook Air? Model, purchase date, Operating System, last installed updates, when was it last working?

Try shutting down the device, perform and SMC reset and then try booting it up. https://support.apple.com/en-au/HT201295

Failing that, I would try and boot it to the Recovery Partition. https://support.apple.com/en-us/HT201314
If that works, reinstall macOS/OSX. (Try first without easing the Hard Disk)

Of course your other option is to speak with Apple support, either over the phone or in store (book a Genius Bar appointment). If there is anything that Apple needs to repair and it is out of warranty they will provide you with a quote (unless it is a really old device).

Let us now how you go.

This sounds like the infamous, I cannot locate my bootstrap. Did it just go through an update that failed or did not complete?

Boot to Target disk mode and use a firewire to fireware cable to back up the data, usually drag the Users account name to a Drive using a machine between the one that will not boot and the external drive.

If it failed an update then you should be able to boot to the drive selection and select the Macintosh HD, not the VM the upgrade / update created.
If it just plain old will not boot::::
Then try to boot from an image stick. If it boots then the problem is something is missing from the OS boot routine. You can either use the image stick OS to try and locate the problem or just reimage the device.

Hope this helps

I'm curious as to how the user made it here to ask that sort of question? Does he not have support on site?