It's ok it happens! ... you can now undo a cancellation and restore a reservation you have accidentally cancelled.
Choose Cancellations from the 'reservations menu',
Search for the reservation
Click 'Undo' beside the reservation you wish to restore.
Select the rooms you wish to restore
Click Undo Cancellations
The reservation will now be displayed on the diary screen (unallocated) so you should drag the reservation into the required room.
To avoid the cancellation list being too long, if you do not specify a booking id, guest name etc when searching, only reservations that are completely cancelled will be shown in the results. If you wish to undo the cancellation of one room in a reservation that is not fully cancelled, enter the id or guest name and reservations that are partially cancelled will be shown in the list.