Ann sent a photo and a question:
We planted a hemlock and it's never done anything until now, when it's all turning brown and the needles are falling...
The hemlock looks to me like it dried to death.Small root ball, top-heavy tree. The gardener waters but doesn't notice the actual root zone is never getting the water, way back under the branches.
I doubt there's enough live foliage left for a comeback, and if there is it'll be a very slow, long drawn out comeback. If you can send me photos of what the trunk looks like where it goes into the ground, I'm betting I'll see it's also planted deep, something growers do all too often in the last stages before selling them off.
Between having a root ball narrow enough to be covered by its own foliage --- how the heck is water supposed to reach those roots?! -- and main roots retarded in growth because they're too deep, these trees don't have much chance. We have had to put weeper hoses way under the plant, to get a hemlock to take. More than once when we were called in after the planting -- but earlier in the problem than appears to be the case with your tree -- we found a root ball so dry we had to drill holes in the root ball to get it to accept water.
If you bought the hemlock from a garden center that gave you a guarantee, take it back now if it's still guaranteed. Get a new one. Then find the flare before you plant it (the widening base of the trunk were trunk meets main roots) and plant it only that deep. Make a watering ring (a raised levee maybe 2" high) around the root ball-plus-one-foot (so, make a ring 3 feet in diameter for a root ball that was one foot in diameter plus a foot all round). Then keep that ring regularly filled with water so you are certain the root ball and the soil right around it is moist.