What does John 20:12 mean?

Peter and John have come to see Jesus' empty tomb, after Mary Magdalene ran to tell them about it (John 20:1–10). Inside, they saw Jesus' empty grave clothes and a carefully folded face covering. After this, they seem to have left without much discussion, and Mary is alone. When she investigates the crypt, she sees two angels.

Most encounters with angels, in the Bible, strike immediate terror into those who see them (Luke 1:11–13; 2:9–10; Acts 10:3–4). John gives few details, but he makes no mention of Mary's fear. Most likely, these angels are appearing in a nonthreatening form, much as they might have done when visiting Lot in the Old Testament (Genesis 19:1–3). Peter and John probably said nothing to Mary when they left, so she probably assumed these men were sitting inside the tomb when everyone arrived. Her sad, apparently calm response to their question suggests the same idea (John 20:13).