I'm sorry to hear that you didn't received a reply on your e-mails.
I've checked your booking. There is no cancellation for one of the flights in te history of your booking. Delta Airlines took over the ticket. When an airline took over the ticket we can't open it anymore. Please contact Delta airlines to check what they offer. When they inform you that a full refund is possible please ask them to this in the booking or let them sent an e-mail. We may not request a full refund without their authorization.
Vliegwinkel Customer Support