Check your email account that is associated with your league account. They should have sent you an email telling you exactly why your account was banned. Sometimes, the email is mistakenly sent to the spam/junk folder, so be sure to check there as well. If you don't see an email, then go ahead and submit a ticket to Player Support, requesting that they tell you why your account was permanently suspended. They will be able to tell you what rule violation occurred on the account, or if it was locked due to a potential account compromise/financial discrepancy.
If you were banned for any sort of toxicity, there would be a Reform Card displayed when you tried to log into the client. It would have the chat logs of recent matches in which you were reported for you to review. However, the reform card and chat logs are only there for cases of toxicity (things like flaming, verbally abusing teammates, hate speech, etc) and would not be there for other rule violations.