If the check was never cashed the bank would not have a copy. If the check was not cashed then the payment has not been made.
If the bank does not have a copy of the check----what do they have? Do they show a payment made for the amount you owed to the IRS? When you pay a bill by paper check the bank will have an image of that check---front and back. Check again with your bank---maybe the check did not make it to the IRS.
if the bank has no record it got lost in the mail or somewhere and therefore you still owe.
in the future, the best way to prevent this is to use an iRS online payment option another option is to send a check by a method where you get proof of delivery. I suspect that very occasionally a paper check gets mishandled at the IRS and lost which means its records will show it never got it.