Slack is experiencing a worldwide outage causing problems sending messages, editing messages, and channels displaying a blank screen. The Slack outage began at approximately 10:10 AM EST and is predominantly affecting users in the USA and Europe. Slack states that they are aware of the issue but have not explained what is causing the problems. Slack status page shows widespread issues across the entire service, though, Slack says they are working to get to the bottom of the problem and will update users again in 30 minutes of time.
Source: https://www.bleepingcomputer.com/news/technology/slack-outage-causes-lag-message-errors-blank-screens-worldwide/