Window's Slack is frozen

It seems its a bug that is affecting some Windows 10 users and most likely due to the Windows notifications. The temporary solution as per Slack support is

1. Ensure that you're signed into at least one team.
2. Completely quit out of Slack.
3. Hit the Windows key and R to bring up the Run dialog.
4. Type (or copy and paste) slack://notReallyWindows10 and hit Enter.

Or login to slack via the web browser.

Try the following: Menu > File > Preferences > Notifications, scroll down to "Deliver notifications via..." and change it to "Slack's built-in notifications". Restart Slack and the problem should be gone - helped in my case.