We are sorry to hear about your lilacs. Without any green leaves, it sounds as if they are dead.
We can't really tell you why. Did you see any symptoms on leaves last year? Were they fully leafed out with normal size and color?
Do you see any gnawing damage at the base of the plants? Give a little upward tug to see if perhaps the trunk has been severed from the roots. Voles (not moles, who only eat worms, grubs and other invertebrates) eat plant material from just underground.
While for most shrubs it's suggested that you can prune them down by a third, lilacs, if their roots are healthy, can take a hard pruning and and return. They look rough and sparse for a year or two and you may loose blooms during that time, they do come back.
Regenerative pruning usually suggests taking a third of the oldest, largest, or poorly growing branches out each year over three years for a slower renewal.
Here is our page on pruning: 
https://extension.umd.edu/resource/pruning-shrubs-and-hedges-home-garden/


Christine