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We can't tell you for sure what might have caused this. Environmental issues (cold damage, drought, sun scorch on trunks) have been known to cause this to happen, but typically on young plants, thin-barked trees, or those that are borderline hardy. This doesn't apply to either the honey locust or the lilac.
To see if they are still alive, you can scratch the bark on a small twig with your thumbnail or a knife. You want to choose a small twig that grew in the last year or two, so the bark is still thin. Scratch just hard enough to remove the bark, not enough to cut deeply into the tissue. Living twigs will feel slightly moist, and will have a thin layer of bright green just under the bark. If it is dried, brittle, and brown, then the twig is dead. You should check twigs on different branches and different parts of the tree so that you are getting a representative sample.
I would advise giving both plants more time before removing them. The weather in our area this year has caused many plants to be delayed in coming out of dormancy. Honey locusts seem particularly slow for some reason.
If you want to take pictures of these plants for more diagnosis, you can upload them to this site, or email them (one at a time) to firstname.lastname@example.org. You can also cut samples and bring them into us for examination. Our hours are 9-3 Mon-Thurs, 9-1 Friday, 9-noon Saturday. Our address is 222 North Havana, Spokane WA. We are just south of Avista baseball stadium at the Fairgrounds.
Thanks again for your question, and let us know if we can answer anything else.