Unfortunately as we are only community members there is not much we can do as we can't look into the billing history and what not, the best thing to do would be to keep trying to contact support either using the live chat, found towards the top right of this page, or phoning them.
One thing I would recommend checking would be your emails. When downgrading/upgrading plans you will receive confirmation emails, check to see if you received one of these emails as you may have accidentally downgraded.
Sorry I wasn't of much help and I hope you get everything sorted!
I hope this helps, Feel free to ask any other questions 🙂