It depends on the type of voucher you have.
If you're the original purchaser and have lost your e-voucher, please get in touch with Virgin Experience Days using this contact form and they'll be able to help you.
Physical Virgin Experience Days' vouchers are valuable documents that if lost, usually cannot be replaced. However if it hasn't been used or redeemed, and they are able to locate it within their system they may be able to help. You can find help on this page of the Virgin Experience Days site.