Visa giftcard denied?

For the past few months I've been using a $50 Visa giftcard that I bought for iTunes and the App Store. It was working fine until a few days ago. I tried to simply update an app on my iPhone 5S running iOS 8.4 and it said "Verification Required." So I clicked continue and entered my password. All my billing information popped up. At the top, it said "There's a billing problem with a previous purchase. Please update your payment method." in red. Everything was already entered except for my security code, so I re-entered it. Then I clicked done. Then it said "Your payment method was declined. Please enter another payment method." I was puzzled. I checked the balance on my card, and there was still money on it. Then I tried re-entering all of my information, but that didn't work. I tried going on my computer to buy a song, and the same thing happened. I tried entering all my payment information on there. Still nothing. I tried looking up some solutions but none of them worked. Any ideas?

Thanks 🙂

- 32GB iPhone 5S running iOS 8.4.1