Ive cancelled my account but still been charged! :( Please help !

Hi !

I cancelled my Prezi account, prior to one month passing so I didnt think I would be charged.
Ive just got charged for one month's service, additionally I cannot log into my account as I have cancelled it!

Please help, I am disappointed that even though deleting my account, I have still been charged.

What can I do in this case?