Hello Bonnie, I would check to see if you discover a crack or tear in the trunk on the affected side, or if there has been recent feeding on that side by rabbits or other wildlife. Branches and leaves failing on one side could also indicate the roots and or trunk on that side of the tree have declined. Check to see if there are buds for next year on the impacted side of the tree. If not, that branch is likely dead. Another way to tell is if the branches and twigs just snap off because they are dry and brittle. If the branch is indeed dead, there is not much that can be done except prune it off. You can also contact a local certified arborist and they can assist you in determining what happened and any next steps.
Sincerely, Richard Hentschel
