2 Lastly, “when she had thus said, she turned herself back, and saw Jesus standing, and knew not that it was Jesus. Jesus says unto her, Woman, why do you weep? Whom do you seek? She, supposing Him to be the gardener, says unto Him, Sir, If you have borne Him hence, tell me where you have laid Him, and I will take Him away. Jesus says unto her, Mary. She turned herself, and says unto Him, Rabboni, which is to say, Master.” Let no one speak ill of the woman because she called the gardener, Sir (domine), and Jesus, Master.
For there she was asking, here she was recognizing; there she was showing respect to a person of whom she was asking a favor, here she was recalling the Teacher of whom she was learning to discern things human and divine. She called one lord (sir), whose handmaid she was not, in order by him to get at the Lord to whom she belonged. In one sense, therefore, she used the word Lord when she said, “They have taken away my Lord; and in another, when she said, Sir (lord), if you have borne Him hence.”
For the prophet also called those lords who were mere men, but in a different sense from Him of whom it is written, “The Lord is His name.” But how was it that this woman, who had already turned herself back to see Jesus, when she supposed Him to be the gardener, and was actually talking with Him, is said to have again turned herself, in order to say unto Him “Rabboni,” but just because, when she then turned herself in body, she supposed Him to be what He was not, while now, when turned in heart, she recognized Him to be what He was.
Source: Tractates on the Gospel of John (New Advent)