Where do I begin? Need help in getting neighbor to repair his boundary retaining stone wall, which has become a dangerous nuisance to my property.

Stone boundary wall is deteriorating onto my backyard property below. Wall is holding up a small residential/commercial parking lot, which has a significant tunnel of a hole that you can see from my backyard. Large stones have and continue to fall into my backyard. The hole is becoming significantly larger with each precipitation. Extra large water runoff into backyard. Backyard ground is becoming soft and pleasurably unusable. Looks as if a possible sinkhole is looming. REPAIRS are needed. I have attempted contact with the owners to no avail. Owners are absent on the property. Property is a small apartment building rental, which uses the commercial parking lot for its tenants.