You may be getting long delays and 'server not found' messages trying to connect to many major websites today, Friday 21 Oct 2016. This is probably because Dyn.com, a big internet provider is under attack. The BBC does not use that server (nor does GreenNet), so should be unaffected. However, it reportedly affects The Guardian, Netflix, Spotify and Twitter among other services. People are posting updates about the situation to Twitter, which isn't going to inform anyone who can't read Twitter.
Here's the Dyn.com status page. It appears (from here in London at 7pm BST) that they are overwhelmed with denial-of-service traffic, so only about 0.3% of lookup requests to them are being successfully answered. The domain name system is designed to 'cache' so this may result in some periods when a website works, and others when it doesn't.
You can use the following information to add temporary entries to your HOSTS file (at your own risk):
184.108.40.206 twitter.com 220.127.116.11 abs.twimg.com pbs.twimg.com 18.104.22.168 t.co 22.214.171.124 theguardian.com www.theguardian.com 126.96.36.199 assets.guim.co.uk 188.8.131.52 i.guim.co.uk 184.108.40.206 github.com # alternatives twitter.com. IN A 220.127.116.11 twitter.com. IN A 18.104.22.168
You might also be able to find similar information by consulting cached ISP records (one web tool for doing this is here). Or you can change your DNS servers to OpenDNS, which holds onto cached records for longer even when it can't consult Dyn,com. The OpenDNS servers are always 22.214.171.124 and 126.96.36.199.
These are some of Dyn's DNS servers that are being attacked:
ns2.p34.dynect.net. IN A 188.8.131.52
ns3.p34.dynect.net. IN A 184.108.40.206