The FTP site undergoes routine maintenance each evening (Eastern Standard Time). Try again the next morning, waiting until the next morning will allow you to access the data.
In addition, sometimes users have settings on their web browser that prevent access to FTP sites. Contact your computer administrator or use online resources to find a solution. On Windows®, make sure that the following is activated: Settings --> Control Panel --> Internet Options --> Advanced tab --> select “Use Web based FTP” check box and/or “Enable folder view for FTP sites”; Click “OK”; and restart your browser.