You can find all cancelled reservations in Reservation View, Cancellations tab:

It is possible to open such reservations from the search bar (Searching for booking or a guest).
There are two ways to restore a cancelled reservation, guests or nights:
- to restore whole cancelled reservation, simply change the reservation status from Cancelled to, for example, Confirmed:
 

Then, when you see the restored information, save the changes.

- another way, which gives you a possibility to restore not only whole reservations but selected nights or guests is to show all cancelled data in Overview using Show All option:
 

Then restore guests or nights using the restore icon:

Then, save the changes. After that all restored information will be available for you again on Bed View:

See also: