Usually you can setup an alternate 2FA like TOTP, a recovery SMS, or something else you can use to prove who you are. And then once you're into the site, wipe the current passkey and setup a new one on your current device.
I cannot think of any innovation that has required more defensive pleading for people to use it than passkeys. When a HN article has to be titled "passkeys are not a trap", the concept has already lost.