Ah lads, this one has been gone through before so many times. There should be a sticky for it.
If you lose the reference number, you can ring Ryanair and they will give you the reference again once you have some details to hand such as the flight date, flight number, possibly your credit card number, etc . . .
They don't charge for it.
Well you have to pay for a phonecall and you have to asborb the opportunity cost of your time spent on the phone, but that could be offset by the value of the Ryanair operators time . . . whatever . . . you ring the number, they give you the reservation code, no money changes hands.
I have turned up at the airport with the reservation details and all they are interested in (not just Ryanair) is my name - which they use to book me in. All the same I wouldn't risk turning up without the reservation details printed out in case it took too long to track down my details or even worse - they couldn't find my details.
z