This might be a permission cache issue in your browser, so clearing your cookies for the site and logging in again can sometimes resolve.
If you are certified and logged in, and it is not a caching issue, you will need to contact Sitecore to fix your account. It is possible that the association to the permission for downloads has been altered at some point.
Also, make sure that the account you are logged in with is the same account email you used for certification. Otherwise, your certifications may not be associated to your account. You can check for your certifications on profile.sitecore.net