I was logged in with 3 different accounts (all chars in IF) to trade money and alchemy pots to this character. Booom! 30mins later account was closed without any explanation.
Your account has never been banned. Most likely, you (or someone else) tried to log in too many times with an incorrect password. This causes a temporary IP lockout. Wait 30 minutes to 1 hour and try to log in again, or if you cannot remember the password, you can reset it at https://en.nostalrius.org/password.
If someone was trying to hack your account by trying common passwords, then it looks like they were unsuccessful.