If you have got that far through the transaction, there probably is a PNR, but you simply haven't been told its locator. So one option might be to call and try to complete the booking on the phone.

Otherwise, presuming that the transaction has not completed and the seat will be released because it wasn't confirmed, you may not need to leave it for hours before trying again. You might be able to get that seat again after 30 minutes or so.