Why has my iTunes gift card suddenly stopped working?

I received a £25 iTunes gift card for my birthday in November and I reddemed it on my iPod touch. I had purchased a film for £6 shortly after Christmas using it and then I decided to wait for the new Disney film Frozen to come out so that I could use the remaining money on it to buy it. However, when it came out at the end of March, I tried buying it but then I saw that the gift card had been taken off my account. Usually it showed that I still had it in the iTunes store at the 'Redeem' section but it didn't have any signs of ever being there. I tried re-redeeming it but it just said that that gift card had already been redeemed.

The only purchase since I reddemed at first was that film I mentioned earlier. I understand that gift cards only expire 2 years after they have been activated so I have no idea what has happened. It's a great pity to lose all the money that was still on it and I really wanted to get that film!

Could someone please help me understand what is going on and if I can still get it to work again???!!!!! 😕😢😮