I have a fairly new (18 months) company account.  Since the last reconciliation the beginning balance has decreased by $2500.  I've undone a few months reconciliation find where the error begins, but went back before any transactions of that amount and the discrepancy is still there.  I've run a data verification and all was well.  Audit trail was turned off. No General Journal entries at all in this file.  

It's as though the account beginning balance changed itself from $0 to -$2500.

How does this happen? What is the easiest way to fix it?