Hi John,

Did you water them during drought? Sometimes trees and shrubs will show delayed impact from catastrophic conditions and suddenly die.

Here's a couple good pages on bacterial blight:

https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-bacterial-blight

http://plantclinic.cornell.edu/factsheets/lilacbacterialblight.pdf

If you can snap off the ends of branches you know they're dead. This page may help you narrow the cause:

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/lilac/

These look like very old plants. If you want to try to keep them, there's no harm in dramatically cutting the shrubs back and thinning out the oldest branches at ground level. Lilacs are tough. They may come back.

This page discusses how to do that. While now is not the best time -- usually this is done in late winter -- you have nothing to lose by trying this now.

https://planttalk.colostate.edu/topics/trees-shrubs-vines/1729-renewing-shrubs/

Or you could cut your losses and remove them. If these are in shade, they will not bloom well again.

Good luck.