my MacBook air doesn't turn on at all

Does the MacBook Air charge, if you connect it to the power adapter?

Has the battery ever been replaced?

If you have tried all steps mentioned in this related article and your MacBook Air still does not start, you should contact the official Apple Support about that.

If your Mac doesn't turn on - Apple Support

Use this link as a start:

Get Apple Support Mac

Or find an authorized Apple Service Provider next to your place:

Apple - Find Locations

Mac Repair & Service - Apple Support

Depending on the age and use, it might be a defective battery.

But, it also may be another hardware issue, which can only be found out by diagnosing your MacBook Air.