Answering respectively: it's lost; no; same way you reset passwords; depends what you mean "what if" - if your passwors manager is already authenticated and doe

Answering respectively: it's lost; no; same way you reset passwords; depends what you mean "what if" - if your passwors manager is already authenticated and doesn't require reauthentication, then yes, it could be used to login. And to answer your additional question, yes, I suppose you would either need to install your password manager or use whatever alternative 2FA login you have.