It looks like it got scorched. I grow avocado
trees in S.W. Western Australia where the sun can get into the mid-40s celsius in summer, and avocados don't like it. Frost will also scorch them, with similar results. They are a sub-tropical sub-canopy tree, and don't like extremes. Mine are grown in the shade of other trees.
On the bright side, a one-off scorching will cause the leaves to go brown as yours have, then black, then fall off. New shoots soon follow if conditions return to normal, so they're not dead. But if they remain in the same extreme that caused this in the first place, they will give up and die.
What is your climate? Micro-climate?