There are three likely causes: corrupted firmware, damage to the internal storage, or a fatal hardware failure.
First try starting it up in fallback Recovery. Connect your MacBook Air to mains power, open it up, hold the Power button for 20 seconds to ensure it has shut down, leave it half an hour in case it needs to charge, then press the Power button twice in rapid succession, keeping the button held with the second press. Ideally, it will then display a message that it’s entering Options. Keep the button held until the Options screen appears, then use tools like Disk Utility to discover what happened.