When something's wrong
I cannot sign in
Sign-in problems, and the one that looks like a sign-in problem but is not.
Cause and fix
Wrong method. Signing in with Google when you originally used email creates or opens a different account. This is the most common cause and it looks like data loss. Try the other method. See sign in.
Forgotten password. Use the password reset. If you signed up with Google or Apple there is no NewTwos password, and the reset will not help.
Apple hide-my-email. If you used it, your account email is the generated relay address, not your real one. Support cannot find you by your real address.
Two-factor code not accepted. Authenticator codes are time-based, so a device clock that is off by more than a minute makes every code fail. Check the clock first.
Lost your second factor. Use a backup code. Without one, recovery is deliberately difficult, which is what 2FA is for. See passwords, 2FA and passcodes.
Signed in fine but everything is empty. That is the two-accounts case, not a sign-in failure. See my things are not syncing.
If it was an anonymous account
An account with no login exists only on that device. If the device is gone, so is the account. There is no recovery path, and this is why creating a login early matters. See turn an anonymous account into a real one.
What to send support
The email you believe the account uses, which sign-in method you used originally, whether 2FA is on, and the platform.