Professor Snape did not continue.
Erwin already understood what he meant. His parents were dead, killed by a person without magic who nonetheless wielded a formidable magical weapon. It was just a stroke of terrible luck at the time. His parents had gone to protect Lily Potter and were targeted by the Solent Family, who seized the opportunity to strike. Snape and the others were stretched too thin to provide protection.
Snape said, "It is my fault. If I had gone to protect your parents, they might not have died."
Chapter 187