If you are a Windows XP user, this operating system is unfortunately no longer compatible with Ankama Shield for security reasons as it is no longer being updated by Microsoft.
If you use another operating system, the service may be unavailable because of your connection or because you're using a proxy. This type of problem is generally caused by Internet Explorer.
If you are experiencing connection problems when using that browser, you should go to Tools > Internet Options > Connection > Network settings and untick all the options.
However, if you are using a proxy, you will need to tick "Automatically detect connection settings" or manually enter the address of your proxy server in the "Proxy server" field.