Mail notifica tions
Stopped seeing notification banners when you receive new emails? In System Preferences’ Notifications pane, select Mail on the left and change the alert style (to None, say), then back to Banners. If that doesn’t help, restart in safe mode (as described earlier), open Mail there and try sending a test email to yourself. If notifications now work, restart in normal mode and try sending another test email.
The act of changing notification preferences sometimes kicks the Mail notification system into action.