How do I get my money back for a failed payment on an iPhone?

So today I was about to purchase a computer and I clicked “split the amount” so I can pay with two cards, because I am so sure on one of them it was 800, and I wanted to pay the rest with my other card. But the error appeared and it was saying something like “failure of payment”, so I gave up. But then I checked with bank the amount on the card with 800 and they are gone. I think some sort of mistake appeared but you just got all of my money from my card and I would like for you to check and give me my money back.

[Re-Titled by Moderator]

Original Title: Return my money