TANs (Transaction Number) can only be used for personal transactions. Your bank will never ask you to enter a TAN for prize draws, security updates or to refund money to your account. Hence, please always use caution if you are asked for your bank details or a TAN when you are not actually making a transaction.
If you ever suspect that something is not quite right with your online banking account: Block your account. To do so, either contact your bank directly or call or 24 hour emergency security helpline at 116 116, calls to which are free across the whole of Germany. This number can also be called from abroad.