A FIDO2 credential can be used for passwordless authentication, so long as the authenticator performs user verification (e.g. requires a PIN). This counts as 2FA because it's something you have (the authenticator) plus something you know/are (PIN/biometric).

It can also be used as an second factor for traditional username/password auth (with or without user verification).

Losing a passkey is no different to losing any other credential: you need another method to authenticate or recover your account.