The description you provide does not give much hope as far as I can tell. The plant should have leafed out long ago. You might try scraping back the bark on some small branches to see if there is any green healthy tissue underneath. Also bend the small branches to see if they are still supple or if they have become dry and brittle. There is little you can do even if it is still alive except water during extended drought spells.
There are several possible causes of a mimosa tree's demise so it is difficult to speculate. Your County Extension Office may also be able to evaluate the current condition of the tree and perhaps suggest some possible causes.
I am indeed sorry to hear about your tree. Thanks for the question. Please stop in again soon!
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