If they aren't showing in your account, then they are gone. Vouchers can be ended early for many reasons, and often are. It's in the terms and conditions. If you receive a voucher, you are best to use it as soon as you can, in case ebay ends it early.
You can't get them back once they are gone. If you don't buy very often, ebay will throw them at you again to encourage you to buy again.
