I wouldn't give up on it until you try these things. But first make sure to wear gloves because it can be toxic to the touch & give your skin a bad reaction.
1. Stem Check - Feel one of the smaller branches to see if it is still flexible. If there is a flexible branch that doesn't feel dry & breaks easily because it is bending gently without breaking, take sterile pruning sheers to clip a section off cleanly & look to see if the inside center still has any indication of life. It is usually green or a bit wet instead of dry & hollow. If there are no living branches the tree could have pulled its lifeforce back to the trunk & roots in despiration. Or, it may be dead.
2. Offer emergency support - to see if it leafs back out. Till the soil without harming the roots around the base of the tree and add some potting soil & a fertalizer that is appropriate for this particular tree. Water that in thoroughly & provide hand watering daily for the next few weeks to see if any new leaf buds form on the tree.
3. Root check - If you push a little on the trunk do the roots feel solid like they are griping the ground? Many plants release their grip on the soil when they die &/or their roots begin to rot & the tree will be noticeably loose in the earth. This means it is likely dead in hard packed soil. If they are still solid the plant may be close to dieing, but salvage able with emergency care.
4. Check thoroughly for pests & pathogens - check the dead leaf debris, trunk & soil for evidence of something that hurt or killed the tree. Is the bark coming off the tree? You would also need to treat for these problems, but if the tree is critical the treatment might take it over the edge. If you care about it a lot you could do a soil test to see if the other plants are putting out a toxin that is killing it. Make decissions accordingly as these things could spread & effect other plants.
If after 2 to 3 weeks of plant ER & TLC it doesn't show signs of life, I would say the tree is likely dead instead of dormant, because of the time of year this happened & you can remove it.