Without seeing your lilacs or analyzing your soil, it's difficult to know what the problem is. It sounds like it's either suffering from disease or isn't getting the proper nutrients. In any event, it won't continue to come back without good foliage.
My advice would be to start by a process of elimination. Is the plant getting everything it needs? It should be growing in full sun in soil that has adequate moisture and drainage. The soil should contain a good balance of nutrients (nothing in excess or severely lacking) and have a fairly neutral pH. Examine the bush for insects (aphids, scale, weevils, borers), and if necessary consult with a horticulturist for a proper diagnosis. For now, remove the flowers immediately after they bloom so the plant's energy isn't wasted on producing seeds and cut down the dead branches and see if you can get some new sprouts from the roots.
About The Author: Ellen Brown is our Green Living and Gardening Expert. Click here to ask Ellen a question! Ellen Brown is an environmental writer and photographer and the owner of Sustainable Media, an environmental media company that specializes in helping businesses and organizations promote eco-friendly products and services. Contact her on the web at http://www.sustainable-media.com
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