Can not start up my Mc book air

Hi ajawadi....!!

Try to do a Reboot...

Shutdown your Mac completely. Then press the power button and immediately press the option / alt key for few seconds until you are presented with a white screen with different hard disks to choose.

Please select the Macintosh HD (Or else your personal named HD) and boot into OS X. Then login into OS X. Then go in system preferences and select Startup Disk and select the Macintosh HD (or else your personal names HD) and press Restart. See if this resolves the issue.

If that doesn’t help then go in recovery mode by pressing cmd + R and select disk utility to repair the startup disk.

Hope this helps.

Also, you can try SMC or else a NVRAM reset.

Resetting the System Management Controller (SMC) on your Mac - Apple Support

How to Reset NVRAM on your Mac - Apple Support

If those resets fail then you try to do a safe boot........OS X: What is Safe Boot, Safe Mode? - Apple Support

If all this fails then you should assume that your disk is failing and you should take it for a repair.

Hope this helps.....

You should press the alt / option button after pressing the power button when you hear a chime.

If that doesn’t help then try my other suggestions and let me know......

Did you follow my suggestions in my second post....??