Change your password straight away. How you do that depends on how you signed up:
Signed up with an email address — change your EQUALS password.
Signed up with Google or iCloud — your login goes through them, so changing an EQUALS password won't help. Change the password on your Google or Apple account first, and contact Google or Apple support so they can secure it.
If you use that same password anywhere else, change it there too. Then report an issue with your =username so we can check the account and help you secure it. If you can't get in at all, tell us the email address linked to the account and we'll take it from there.