Thank you for contacting Toronto Master Gardeners. Camperdown elms are susceptible to Dutch elm disease and it looks like this is what has caused the leaves to drop on your tree. An arborist can confirm whether this is the case. Unfortunately if your tree is infected with Dutch elm disease there isn’t any thing you can do about it. The tree will have to be removed.
For further information, please see:
To find a certified professional arborist to help you with a tree problem, visit the Ontario branch of the International Society of Arboriculture here.