If your case has been closed for less than 6 months you can re-open it yourself! Click on 'My cases', look for the closed case you wish to re-open (this will be faded and beneath your open case files). Select the case and look for the 'Re-open Case' button.
For any longer than 6 months, please Submit a request.
Your case will update itself if the company has responded while it was closed. This means any messages will be in your case file if you reopen it.