Sometimes appointments are deleted from the calendar inadvertently. The Appointment Recovery feature can reinstate deleted appointments as well as inform by whom and when the appointment was deleted.
Note: Only appointments that have NOT been invoiced, estimated, DON'T have chart or procedure notes created or are NOT for a birthday special can be deleted, otherwise you'll get an error message and the appointment will remain in the calendar. 
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To recover a deleted appointment, follow these steps:
In the sidebar go to Schedule -> Appointment Recovery.
Find the appointment that you want to restore to the calendar and click Recover -> Yes.
The appointment will again be visible in the calendar.
Video
Content Timelines per Minute:
00:00 - 00:20 - Introduction
00:21 - 00:37 - Deleting an appointment
00:38 - 1:16 - Recovering the appointment
1:17 - 01:26 - Checking that the appointment is recovered