It all depends on what caused the tree, or branch, to fall on your neighbor's property. If the branch became detached from the tree due to the storm, the damage would be considered an act of god and should be covered by your neighbor's homeowners insurance
policy. If the branch became detached because it was rotten or diseased and you were aware of the issue, you may be considered liable. I would recommend contacting your insurance company to let them know about the incident and they can direct you on how to proceed further.