Work down this list. Most people are back in within a minute or two.
Your password is not accepted
Use the forgotten password link on the sign in page. You will need access to the email address on the account.
Your authenticator code is rejected
Almost always your phone's clock. These codes depend on your phone's time agreeing with ours. Turn on automatic date and time, then try the current code.
Otherwise make sure you are entering the code showing right now; they change every 30 seconds.
You have lost the phone
Use a backup code. At sign in, choose to use a backup code instead. Each works once.
Once you are back in, go to Profile, turn two-factor authentication off and on again with your new phone, and save the fresh backup codes.
Lost the phone and the backup codes
If you also enabled email sign-in codes, use those instead and you are fine.
If not, contact support. Be prepared for this to take longer than you would like and to involve proving who you are. That is not obstruction: an account recovery process that is easy to talk your way through is a way in for anyone who can impersonate you convincingly, and the findings in your account are worth protecting.
It is much easier to avoid than to resolve, which is the whole argument for saving your backup codes when you first see them.
You cannot reach the email address on the account
The hardest case, because both password resets and email codes depend on it. Contact support.
The lesson to take from it: if the account uses an address belonging to someone who has left, or a mailbox nobody reads, change it before it becomes a problem. See Changing your email address.
If a colleague is locked out
An Owner or Admin cannot reset another person's password or two-factor settings; those belong to that person. What they can do is remove and re-invite them, which gets a working account back quickly if the old one is genuinely unreachable.