MacBook Air forgotten password
There are two ways to go... The way where you get your data back, and the way where you lose everything. Assuming you were using FireVault for your disk encryption,
Reset using your Recovery Key (FileVault must be on)
If FileVault is turned on and you have a FileVault Recovery Key, you can use that key to reset your password.
At the login screen, keep entering a password until you see a message saying that you can reset your password using your Recovery Key. If you don't see the message after three attempts, FileVault isn't on.
Click the arrow button next to the message. The password field changes to a Recovery Key field.
Enter your Recovery Key, then follow the onscreen instructions to create a new password. Click Reset Password when done.
Create a new login keychain
Or
Wipe Everything
Boot into Mac Recovery Mode ⌘ + ⇧ + R.
Open the Terminal from the Utilities menu
Type resetpassword
A window should pop up to assist you...