When this happened to us, I got on chat with microsoft support. They told me the events would be in my "deleted items" folder and then I could restore them by looking there, selecting the items, and doing a restore. If that works for you, great. I had NO items in my "deleted items" folder and the tech who was on screen with me confirmed it. She had to go get supervisor help, but nothing they suggested worked. Fortunately I had made a backup of my calendar so we were able to restore it from backup. However, they had no idea how the events could have suddenly disappeared and no idea of a remedy. The strange thing is that after I restored all items, we now see 1 calendar event for everything, but if there was a reminder, there are 2 of them, and they are identical. I'd really like to see Microsoft fix Outlook. We keep Office365 for all the other apps, and back in the day, when we both worked at corporations with Exchange servers, Outlook worked well. We love Outlook but it is failing over and over in this and in other ways. I'm disappointed that without resolution, they are satisfied with the level of support.
When the items all disappeared, my wife was searching calendar entries using the search feature and had just modified the entry she was looking for. Sometime between hitting the "save" button for the item and blinking her eyes, everything disappeared. If that is what you were doing when items disappeared, I suggest to NOT use any search features in the calendar until Microsoft supplies a patch of some kind.