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Junipers are very robust trees, as long as other conditions are being met (primarily dealing with health of roots). You should see some of the junipers growing out in the Mojave desert if you are concerned about heat
However as others have pointed out, it is very difficult to grow most bonsai indoors, and for conifer bonsai, almost impossible. Even if you are able to keep them alive, you may lose the ability to maintain them as convincing bonsai. Because they aren't getting adequate lighting, the inner growth may die out, and you may get spindly, sparse new growth with long internodes.
It appears to me that your juniper may have moved on to the big bonsai garden in the sky.
Junipers have the unique ability to stay "evergreen" even when "everdead". However I am not always right about junipers - I have in the past seen some junipers coming in from the field in the winter that look dead to me... but they recover miraculously once they wake up from dormancy. I don't think this is the case with your tree, but if you want to be certain you can always wait another month. If you don't see a bud of new growth anywhere on the tree after a month, it is ready for the burn pile.
However do not despair. I have lost track of the number of junipers I have killed. The key is to never kill the same tree species the same way twice! That means you are learning from your mistakes.