I accidentally erased my MacBook Air

Have you been making regular backups either through Time Machine or some other backup strategy.

If you have been using Time Machine then boot to your Recovery HD, Restart your mac while pressing

and holding down the Command and R keys.

After a couple of minutes you will see a Utilities panel click on Restore from Time Machine,

press Continue.

If you have not been making backups then you may have to reinstall the OS from Apples Recovery Servers

from the same panel select Reinstall OS and press Continue.