Why can I not access YouTube?

I've been using YouTube with no issues until yesterday. Now, whenever I type youtube.com into Safari it sits and does nothing then eventually presents the message "Safari can't open the page" and adds "...because the server where this page is located isn't responding". All other websites are responding as normal.

But YouTube works just fine from my android phone and my iPad Air. The mac mini is connected via ethernet, the iPad via wifi, and both connect to the same router. I've tried turning off my AVG anti-virus package but it makes no difference. I assume it's a setting somewhere, but have no idea what or where.

I also installed Firefox which has the same problem. Any suggestions gratefully received.