You can find similar information on the TfL website here but most of the advice says to contact your card issuer - which won’t help unless your card has been blocked.
Normally what’s happen is that TfL will try repeatedly, but block your card on their side until the payment goes through.
If they stop trying then you can usually force it through in two ways - by logging in to your tfl contactless account (or creating one and adding your card to it) or by tapping it on a TfL card reader.
If your card is blocked then it won’t let you through the barrier, but once they’ve successfully taken the funds and processed it on their side they’ll unblock your card and you’ll be able to use it to travel. This generally takes around 30 minutes