Twitter said the outage was due to "an internal system change" and it's fixing the issue. It said everything should be up and running again soon.

Twitter is back up for some people, and we're working to make sure our service is available to everyone as quickly as possible. The interruption was due to an internal system change, which we're now fixing. We're sorry for the inconvenience and should be at 100% soon. https://t.co/HXOiFVbgPK — Twitter Support (@TwitterSupport) July 11, 2019

Update 11/7/2019 4:13 PM ET: Added Twitter's statement about the issue and to note it's back online for some people.