You need to look at all of the postings to that account, especially things like adjustments which are usually the cause. That means track down every single debit and credit in the posting journal (since day one) that affects that account. If the debits and credits don't add up to the current GL balance you will have to post a correction. And find out: who did it/what they did/why they did it/when they did it/(maybe)how to prevent it -- in order to establish why you need to correct the batch.