Don't panic! But you will be required to pay Dollar, Alamo or National (as appropriate) the full price for your car hire when you collect the car (this is considerably more than the price we charge!). Then explain to the rental company that you do have a pre-paid voucher that you have lost / forgotten. You will need to note your Rental Agreement Number, and contact us as soon as you can on email@example.com giving the above information. We will then send your voucher to the rental company and when you return your vehicle they will refund the extra cost that you have paid them. Note, it is very important in this instance that you do not take a car in a higher category than booked.
Note, with Alamo and National, your voucher is not actually required but you would be required to know the Alamo confirmation number (that is printed on your voucher).