While Archiving, an appointment dropped off my Schedule, however, the appointment appears on my Client’s card under Past Appointments.

It appears that you’ve accidentally marked the appointment
as a ‘NO SHOW’ and that’s why it was removed from your schedule. You can re-open the Appointment card and close it out again
properly:

1.     Go
to the Client’s card
2.     Tap
on ‘Appointments’
3.     Tap
on that particular appointment under ‘Past Appointments’ – you’ll see the red
‘NO SHOW’ banner on the Appointment card
4.     Tap
on ‘Edit’ on top right of the screen and confirm re-opening
5.     Tap
on the appointment again and you’ll see the re-opened Appointment card
6.     Close
out the Appointment card properly
7.     You
will see the Appointment back on your Schedule