How do i use the disk utility if my MacBook Air doesn’t work?

If your Mac starts up to a circle with a line through it

A circle with a line through it means that your startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. 

A prohibitory symbol, which looks like a circle with a line or slash through it, means that your startup disk contains a Mac operating system, but it's not a version or build of macOS that your Mac can use. 

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Use Disk Utility to repair your startup disk. 
3. If Disk Utility found no errors or repaired all errors, reinstall macOS.
4. If you still need help, please contact Apple Support.

Try starting it into Safemode, then run First Aid from the Disk Utility.

Start up your Mac in safe mode - Apple Support