You need the number, but it's not that hard to track down.

After logging onto the RR account, you can get the PNR by clicking on "Itineraries". That page only lists flights you've booked while logged on - but that doesn't sound like an issue in your case. If you booked by a different means, I don't think you'll see the flight listed on the main page either.

Fallback is to call the reservation number and have them look up your PNR by flight # and name. I've done that a time or two.