If you’re still able to access the auction housing and can buy and sell players then it’s not a ban on the account. If the coins are missing, then they were used. The only way to know if they were used would be to go into your binder change it to recent and see if you have any new players.
If you go through your account and you see no new content, you would have to contact live support and have them check the logs to see where those coins went. The thing about currency is it can’t just disappear from the game. It has to be used or removed. if you have no emails and no cases saying that this was removed or you were in violation of any terms of service then the coins were used. They may not have been used by you, but they were used by someone.