I had this happen to me a few times already. Notification Center on the Mac goes completely numb: no more notifications and the list is empty. I’m writing down the fix, so I do not have to google the forums once again the next time.
Open the Terminal, then:
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# this usually fails, as the notification server is already running
launchctl load -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist
# kills and restarts the notification center - fixed!.