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:
# 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!. killall NotificationCenter