Hello

The remaining leaves are heavily coated with powdery mildew fungi. This could be why the leaves have fallen early. Be sure to rake up as many of the diseased leaves as possible and discard in the trash.

https://extension.unh.edu/blog/2018/09/what-should-i-do-about-powdery-mildew-my-lilacs

Lilacs have a few other issues, such as lilc borer insects. These won’t show in you photos, so compare your stems to the pictures/descriptions here-

https://web.extension.illinois.edu/hortanswers/detailProblem.cfm?PathogenID=92

https://davesgarden.com/guides/bf/showimage/6055/#b

https://extension.usu.edu/pests/ipm/notes_orn/list-treeshrubs/lilac-ash-borer

Has the area been flooded or been very dry in the last 12 months? This could kill roots. Remaining live roots will help the plant recover, if you can get the water regulated so that they are moist, well drained.

Consider removing old wood gradually, no more than 1/3 of the large stems in a season, by pruning 2 inch and larger stems back to the ground. This is a “gentler” method than pruning everything back at once. Any completely dead or obviously diseased wood can be removed anytime. See- https://hortnews.extension.iastate.edu/1993/2-10-1993/lilac.html

In spring you may apply a flowering shrub fertilizer to encourage fresh growth.