What was the source of the gift cards?
If you purchased the gift cards from a third party seller (resale gift cards sold at a discount) they are most likely fraudulent cards and this is why your account got locked down, because of fraud.
If you purchased the gift cards using bit coin, they are most likely fraudulent cards and this is why your account got locked down, because of fraud.
If you purchased the gift cards from a reputable third party seller, I suggest you dispute the charge with your bank (paid via debit card) or your credit card company paid via credit card). If you paid cash as a walk in consumer, I suggest you go back to the store you purchased from and speak with them as they obviously have issues with their cards. If you have the actual card, keep it.
When people post on these boards about gift card situations, the situation is that usually these cards were purchased from a reseller at a discount and the consumer is unaware that the card is fraudulent. If this is the case with you, you need to address this issue with the reseller directly.
Since the card was already used, your attempt to use it is considered fraud and, therefore, your account violates Amazons TOC and is closed. Any remaining balance is seized. If you want that balance back, you need to seek a lawyer and/or file with the atty general.