There are a few reasons this could happen. First the database may have gotten corrupted in this case from your customer/vendor list choose utilities, recalculate balance from here you have the option to choose the current customer or all customers.
If recalculate balance does not fix the problem the second thing to check is that beginning balance and year-to-date are both turned on (this should be reflected on the bottom right hand corner of the screen next to current year). To turn either of these options on, from your customer/vendor list choose utilities, edit current status and make sure both of these options are checked.
If this does not help, it is possible that any of the previous years were not closed out or entries were made after they were. In this case, go back to the year that was not closed out and reclose it (from main screen utilities, end of year). Remember each year thereafter must be closed out until you get to the current year.