The server may have been overloaded or felt under attack and thus may have
banned your (or ISP's) IP address.

Usually such bans are temporary. You may want to try again after a day or two,
but this time, try not to put pressure on the server. You might find some
useful hints from <http://forum.httrack.com/readmsg/34153/34143/index.html>

You might want to check if they object to downloading of their site as well.

Regards,
Annajiat