Noel Paton wrote:

Have you tried pinging your DNS server? - this sounds more and more
like a problem with your ISP, rather than one at your end
If you can successfully ping the DNS server, but nothing beyond it,
then it may be that the server is either poisoned or not functioning
properly - and that is definitely your ISP's problem.

This could be checked by manually configuring the system (or router if
applicable) to use a different DNS server.
--
Mike Maltby