While we do our best to make our payment systems bullet-proof, there are circumstances beyond our control that could prevent a payment from going through.
Unless you got a confirmation that money was taken from your account already, you should try paying again. Perhaps you made a mistake when typing in your card number?
If you are sure that your payment information was correct, then it's possible that your bank did not authorize the transaction, for some reason. Keep in mind that we are based in Europe, and your card must support such transactions (for instance, many American prepaid cards do not support international payments).
It may be necessary for you to contact your bank for assistance, because most of the time payment issues are beyond our control. If your bank says that everything is OK on their end, or that they don't see any payment attempts from us (GOG.com or "GOG Limited"), you should definitely contact us using the support form.
Most importantly - if it looks like you did get charged, but didn't get your game, contact us immediately.
Keep in mind, though, that banks normally "block" funds prior to a transaction, so it's easy to mistake blocked funds for actually charged ones. Make sure to double-check if you were indeed charged.