Did you put your card info in to Dropbox? If so you did authorise it - and probably didn't cancel a trial in time. See https://help.dropbox.com/plans/refund
If you DID NOT give Dropbox your credit card info then you need to speak to your card company as somebody has cloned it.
- - - -
Did this post help you? If so please mark it for some Kudos below.
Did this post fix your issue/answer your question? If so please press the 'Accept as Solution' button to help others find it.
Did this post not resolve your issue? If so please give us some more information so we can try and help - please remember we cannot see over your shoulder so be as descriptive as possible!