Thanks for the question.  I am sorry to hear about your lilac.  They are very beautiful trees.  We have been getting numerous questions about shrubs and trees not leafing out and/or not developing flowers this spring.  We believe that the very cold temperatures of last January, coupled with insufficient snow cover and unusual spring weather, are the culprits behind this.  From your comments and the pictures, it would appear that these environmental factors hindered the development of leaf and flower buds, perhaps even directly killing them.  The tree may survive but its growth may be compromised by a lack of leaves.  Because you have such an affinity for this tree, I am hesitant to say much more using only the pictures.  I believe that it is time to enlist the services of a professional, licensed arborist.  As volunteers master gardeners of the University of Minnesota we are unable to make recommendations as to specific business enterprises.  However the following University of Minnesota publication should guide you in this selection:

https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

Good Luck!!