If you're using a DOLLARUA Type A certificate, you absolutely positively do NOT get it back. The agent usually warns you about this when you make the reservation. You might have luck if you ticketed in person and beg to get the paper cert back, but that is against the rules. Now if you try to CHANGE the ticket online, it usually applies the cert to the change too. That's not guaranteed to work though.

I don't have experience with other (refare) vouchers. Do you have a copy of the terms that were on the back of the cert?