Follow the steps below to reactivate an appointment type.
Select Appointments > Configuration… > Appointments Types
Place a check in the "Include inactive types" box to include inactive appointment types in your search results.
Use the Show All button to view all appointment types. Or use the Search field to further drill down your search.
Click OK when “This appointment type is currently inactive.” pop-up displays.
Uncheck the "Make this type inactive" checkbox.