Thank you for your question about your Monkey Puzzle Tree.  I don't know where you and your tree are, but from the looks of the pictures, you have plenty of water, which is something that Monkey Puzzle Trees need a fair amount of (even though they are native to the the lower slopes of the Chilean and Argentinian south-central Andes.)

Had this been a a drought stress issue just this season, you probably wouldn't be seeing the dead branches clear up to the top of the tree.  Because the onset was so sudden, I believe that the cause is either an herbicide that has been (inadvertently) applied to the tree, causing it to die, or the presence in the soil of the pathogen Pestalotiopsis funerea, which has been identified as the cause of the death of others of this species.

I would suggest that you contact a local horticultural expert who can examine your tree and, perhaps, be able to diagnose the cause.  Tragic to lose such a magnificent (and increasingly rare) tree!

Good luck!