This may sound a silly question but are you sure they are dead?
At the minute quite a lot of trees and shrubs are losing leaves in reaction to the heat and drought.
If you scratch the soft bark on a branch with your nail and its green underneath then its still alive and keep watering well until the rain comes

Thanks for your reply I think I can safely say that they are dead have done as you suggest but no life.

I always think of roots if nothing can be seen above ground, you may find something when you take them out, I,d have a good look to see if there is any sign of maggoty things or fungus for example

Thank you Pamg for your help but I,m afraid the lilacs are now on the compost heap couldn't find the cause so have freed the space to somethig else.

Many thanks Pamg for your help but I'm afraid they were dead but no obvious cause so they are now residing on the compost heap I will buy something new for the vacant space.