Email notifications for calender events have stopped working

Email notifications for calender events have stopped working

Hi,
I have several weekly recurring  calendar events set up that send me email notifications 6 hrs and 5 mins before  each event. They have been working well - some of them for years without any problems. However in the last few days they have all stopped. The events are still in the calendar with the email notifications still set. 
Any ideas?

Thank and regards
Jim