Do not worry - if the case has been closed for any less that 2 weeks you will be able to 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 2 weeks, please Submit a request.
What's more, your case will automatically update if the company has responded even while it was closed, so any messages will be in your case file when it is reopened.