Prizes that you've claimed can be found in your settings, transaction history, and redemption history. You usually will also find instructions to redeem them in there, and a link to where it can be redeemed.
If you've won a user-created giveaway and you haven't received the prize, our best recommendation is to reach out to the creator of the giveaway. If the creator of the giveaway refuses to fulfill their prize, Sliver will not fulfill the prizes on their behalf. Your course of action would be to report them to us and we will take action against their account.