Short answer
HubSpot's KB: the desktop add-in is in maintenance mode. Check it's installed, then sign out from the ribbon and back in. Real-time notifications come through the Chrome extension.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Consider the Office 365 add-in if issues persist.
3. Related
Add-in missing and Office 365 troubleshooting.