‘And they say to her, “Woman, why are you weeping?” She says to them, “Because they have taken away my Lord and I do not know where they have laid him.” When she had thus said she turned herself round and sees Jesus standing there, and did not know that it was Jesus.'
Mary did not realise that the men in white were angels. So while the angels were seeking to deal with the source of her distress, she was too overwrought to listen to them. Her mind was filled with the question of what had happened to her beloved Master, and she turned away having automatically answered their question. The two men were irrelevant to her. She did not even ask herself what they were doing there. She was too distraught. This complete lack by Mary of awareness of the presence of angels is another evidence of the authenticity of the account.
So she turned away, not really heeding their words, and saw another man standing there. It was Jesus, but she did not realise it.
It is important to note that this was the opposite of hallucination. In hallucination you believe what you see, however amazing. But Mary was too practically minded to hallucinate. She was actually seeing amazing things and did not realise they were amazing, but interpreted them in earthly, down-to-earth terms. She saw these figures as just men (Mark 16:5) who were there because they presumably had a job to do.
She also only dimly saw the man outside. The day was still just beginning and the light was not good, and Mary's eyes were flooded with tears. She saw but a vague figure standing before her. After all the last thing that she was expecting to see was Jesus fully clothed presumably looking hale and hearty. He would have looked very different from when she last saw Him, a broken bleeding figure on the cross. Indeed His appearance seems to have made Him partially unrecognisable. Resurrection had clearly changed His appearance somewhat as we would expect (Luke 24:16). All this helps to explain why she did not recognise Him.