Radha was five years elder to Krishna. That's why she could not marry him, they say.
Another reason was that he was a prince and she felt that she was just a cowherd, though Krishna never felt like that.
One more reason was that she witnessed some of his miracles right before her eyes and she considered him as a God. She worshipped him in her mind, so she could not develop mundane love.
Nevertheless, their love was beyond these limitations, they were soul mates in many of their previous life times like Sita Rama or Lakshmi Narayana.
Krishna left Vrindavan to Kill Kamsa at Mathura, when he was just ten, never to return again.
Radha married Ayan alias Abhimanyu, gave birth to a son called Shon, lived with them till her husband died, her son got married and left Vrindavan to meet Krishna at Dwarka.
She met Krishna in his palace. Though Krishna welcomed her and treated her with the same love, affection and respect that he had before, and asked her to stay with him in the palace, she was not happy there since he was busy with his duties and he could not attend to her personally.
She left the palace without informing him and walked back to her city. When she was walking in a forest on her way to Vrindavan, she heard the footsteps of someone following her.
She turned back to look at who it was. It was none other than Krishna himself!
Krishna asked her why she left him without even informing him. She replied to him that she felt lonely there since he was busy with his royal duties and decided to go back to her city.
He asked her what she wanted from him. She asked him to play his flute.
Krishna started playing on his flute and she was hearing the divine music with her eyes closed.
She never opened her eyes again.
Krishna could not bear Radha's death and broke his flute as a symbolic ending of love and threw it into the bush.
Krishna began to meditate in the forest to leave his body. A hunter named Jara perceived him to be a deer and shot an arrow at him. Ultimately the poisonous arrow took the physical life of Krishna.
