The Blender.org infrastructure has been under a Denial of Service attack since this weekend. As a result, all official Blender sites are currently down. The team is aware and is working on a solution.
Be careful when downloading from alternative locations, the Blender Foundation provided the following list of official mirrors:
Updates will be posted centrally on this Blender Artists thread.
Update, November 22: It appears most of the issues are over now. After running a 'light version' yesterday, Blender.org has now been placed behind CloudFlare for DDoS protection and I was able to access it again! 🎉