Slack is currently experiencing a massive outage affecting users worldwide. Messages cannot be sent and received, and channel history cannot be retrieved. Slack has restored some of the functionality of the Slack client and can connect again to servers and receive and send messages. Google Calendar and Outlook Calendar aren’t working as expected, and some email notifications are failing to send. Some customers may see improvement in connecting to Slack after a refresh (CTRL/CMD + R), but some customers may also experience degraded performance.
Source: https://www.bleepingcomputer.com/news/technology/slack-suffers-its-first-massive-outage-of-2021/