"This phone was signed out. Sign in again." means this phone's sign-in ended. Your notes are still on the phone; only the account features (the AI assistant, your plan) need you to sign in again.
Why it happens:
- You signed in on another phone and chose to use it instead (the plan covers fewer phones).
- The phone was removed on the website under Phones.
- The phone wasn't used for a long time.
To fix it: open Settings → Account and sign in again.
If none of these was you, change your password on the website, turn on two-step sign-in, and remove any phone you don't recognise under Phones (changing the password signs out other browsers, not phones).