It is possible that somebody from Upwork will intervene and offer you help.
But you really don't need their help. You can do this all on your own if you don't want to wait.
The Upwork freelancer-side interface includes a button next to each closed contract that the freelancer can click on which will then allow you (the client) to CHANGE your feedback.
All you need to do is explain to the freelancer whose contract you accidentally closed what happened, and ask him to click the button.
Then you can change the feedback (written or star ratings) to anything you want.
You can not re-open the closed contract, but you can create a new contract. You can even use the original contract as a template for the new contract.
If it was me, personally, I would just go ahead and do this. It would be faster than trying to do these things through Upwork Customer Service or waiting for an Upwork moderator to help. If you do reach Upwork Customer Service or get advice from an Upwork moderator, they'll probably just provide instructions for how to do these things anyway, rather than actually changing things themselves.
If you have ANY questions about how to do any of this, post your question here. The Forum is open 24/7.
We're all human and make mistakes. However, it's extremely unfortunate when an error can so negatively impact someone else. As Baris stated, the negative Private Feedback that was given to the Freelancer will remain unless Upwork removes it. Potentially that could have devastating consequences to the Freelancer.