The error indicates that the network connection PuTTY tried to make to your server din't receive any response at all from the server. This occurs when there is an issue with the server or mostly it is offline.
a) Make sure that you are able to access any other site, to ensure that there is no issue with your network connection.
b) Check that you have correctly entered the host name or IP address of your server machine and to the correct port specified by the administrator of your server. By default SSH connects to the port 22, but due to various threats, System Admins change the default port to something else.
c) If this fails, consult your System Administrator