Twitter is experiencing a worldwide service disruption preventing users from sending tweets or seeing notifications. Twitter says the outage is caused by an inadvertent change made to internal systems. Twitter went through a similar outage on October 1 when users were having trouble both sending and seeing tweets. The social network also experienced a similar disruption in August when users couldn’t receive 2FA verification codes via phone calls and text messages, making it impossible for users to authenticate if they had two-factor authentication configured to use these two methods.
Source: https://www.bleepingcomputer.com/news/technology/twitter-outage-blocks-users-from-tweeting-seeing-notifications/