Connection failure - "No such host is known"

When I try to connect to my site this is the response I get.

[12:40:53] Resolving host name ""
[12:40:53] Proxy: Resolving host name
[12:40:53] Proxy: Can't resolve proxy host name
[12:40:53] No such host is known.
[12:40:53] Cannot login waiting to retry (30s)...

I managed to connect successfully about a week ago using (as far as I know) all the same settings and the same computer that I am using now. I have been failing as above for the past 3 days. I rang the helpline for my web hosting service and they said they managed to connect to my site using SmartFTP from their computer so had absolutely nothing helpful to say.

Help me please, I'm going barmy!