It might be as simple as your ISP upgraded their DNS servers and screwed up. I experienced similar problems as you. When I could get someone at the help center who could speak English and even know what I was talking about it was massive frustration. I went to OpenDns. They have a free option for use at the basic level. Worked like a charm. I recently checked back with my ISP by recoding the DNS settings back to them.
They had fixed THEIR problem. They had insisted I was to blame for my problem but I went back to OpenDns anyway.
You can check at http://www.dslreports.com/ sometimes because ISP’s are notorious for denying existence of problems at least mine was