After you close your year-end there is no way to re-open it to go back and make changes. The only way to do this is to restore a backup from prior to closing your year (which will wipe out any transactions that you have entered then).
In your case though, there is one thing that you can do.
Set your working date back into 1996 and go under Accounting to General Ledger and select Input Journal Entries. You will want to select the loan account that has the balance in it (be sure to get the right account Long Term or Short Term - check your Trial Balance to see which account has the balance in it) and enter the appropriate amount and click on the Debit button. Next find your checking account (or whatever account was used to payoff the loan) and enter the amount and click on the Credit button. Make sure the date is back in 1996 and record the transaction. This will fix your books in 1996.
In general, you should wait to close your year-end until you are absolutely sure that everything is correct. I normally say to wait until you have your tax return filed. You CAN be recording transactions in the new year - just be sure to use a date in the new year for each transaction. When you close your year-end the program will ignore any transactions dated in the new year.