it was winter damaged ... and it didnt die.. until the heat of summer... it just couldnt process what it needed.. due to the injury ...

cut it flush to the ground or 3" .. the dead part ..

enjoy next year ...

i would not be concerned about the rest of the plant... its a lilac.. park the car on it.. it will be back ...

ken

ps: inspect cut part.. and see what you can find out.. perhaps skin it.. and find out what the damage was .... WAG... freeze/thaw damage on the sun side.. akin to winter sun scald ...or winter wind desiccation ....

I originally thought root rot but winter damage sounds more like it.

I lean toward winter damage too...because as Ken intimated, the lilac is inclined to be the survivor of the atomic attack at ground zero.

In Nebraska they were used as living fencing..border markers, because they were indestructible.