Twitter, Spotify and Reddit, and a huge swath of other websites were down or screwed up yesterday. This was happening as hackers unleashed a large distributed denial of service (DDoS) attack on the servers of Dyn, a major DNS host. It's probably safe to assume that the two situations are related.
Domain Name Servers (DNS) act as the Internet's phone book. Basically, they facilitate your request to go to a certain webpage and make sure you are taken to the right place. If the DNS provider that handles requests for Twitter is down, well, good luck getting to Twitter. Some websites are coming back for some users, but it doesn't look like the problem is fully resolved.
Dyn posted this update on its website: "Starting at 11:10 UTC on October 21th-Friday 2016 we began monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure. Some customers may experience increased DNS query latency and delayed zone propagation during this time. Updates will be posted as information becomes available."
Dyn said that it was still dealing with the problem.