The first thing you should do is lock your card from the Card screen.
It’s worth checking:
This list of popular merchants and the names they trade under. Merchants sometimes appear under different trading names when it appears on your feed and statement. If the merchant doesn’t appear there it might be worth searching the company online.
Subscriptions/Free trials. A free trial might have rolled into a paid service.
Recurring payments. Check if there are any ongoing payments that may have been arranged on a recurring basis.
If you still don’t recognise the card transaction, please get in touch.