My neighbor hired a contractor to come over to remove a retaining wall from my property without me knowing Can I Sue my neighbor for property damage ?

You can absolutely sue your neighbor for trespass and property damage for the cost to rebuild your retaining wall, assuming it did not violate any local building codes and ordinances. Such statutes generally set height limits, may have guidelines for building materials and may also require permits. Before you rebuild check your local building department to determine whether you need a permit. Even if your retaining wall was built without a permit or violated the local building code in some other way, that didn't give your neighbor the right to take it down. They could've complained to the building department which would've investigated and made a determination as to the legality of the retaining wall.