During the last week or so there have been many threads from those in trouble.
They all turned out to have these common factors regardless of the ISP.
They were all running XP with recent updates
They were all using Zone Alarm as a firewall
It seems that one particular MS Security fix update came into conflict with almost the latest ZA, with the result that there was no way the sufferer could go online.
If your problem PC doesnt have both XP and ZA, then don't bother to read on, the rest won't apply.
Temporary cure is to uninstall the update, but this is unnecessary if you go to the ZA website and download the latest Free, Firewall only, file.
This should have a name "zlsSetup_70_483_000_en.exe"
Mrelt check that this is the file you have downloaded, run it so that it will overwrite the existing ZA and "Bob's yer uncle!