what account was originally debited, ie

   dr ???

   cr bank

There was no account originally debited.  The opening balance was entered in historical mode.  I don’t think there is a way to go back to historical mode once it’s closed.  I’m starting to think I might just have to flip it into a write off account.  

if the balance was recorded in historical mode, presumably from a trial balance, and the bank balance is wrong, then so  must one or more other accounts - or you would still be in historical mode

have you printed a sage trial balance as of day 1, and compared it to what was entered in historical mode ?

Even when entering amounts in Historical Mode, debits must equal credits.  If the person who did the setup entered the 50K credit and didn't debit anything, Sage would have automatically debited the "Retained Earnings - Previous Year" GL account.

I would do as Roger L suggested and print a Sage trial balance as of day one and compare it with what was entered in historical mode.  $50K is a lot of money to flip into a random write-off account, so if you can find out which account was originally debited to offset the $50K credit, it will be easier to fix.  My guess is that you'll find your missing debit in the Retained Earnings - Previous Year account.