what did happened to shree radha rani ji after krishna went to mathura?
The most important thing to keep in mind is that Radha and Krishna are absolutely one, and are Supreme God. They can never be separated, even if in a leela They may be apart. When both are omnipresent, then how can They be apart? When both are one in two forms, then how can They be apart? So whatever They do is called a leela; it is a Divine act which is meant to reveal Divine bliss and grace the souls. We should not question in or analyze Their leelas. We should only use them for the devotional remembrance or Smaran. If there is something in Their leelas that seems difficult for us to accept or understand, we should just set aside our questioning intellect and have faith that there is a Divine reason for all of God's actions.
That being said, after Shree Krishna left for Mathura, there was no marriage of Radha Rani. When Shree Krishna left for Mathura, He gave 100 years of virah to Radha. Virah is a state of the Divine love where even though there is perfect union between Radha and Krishna, the feeling of separation heightens the exciting experience of Prem Ras. The revelation of this virah also demonstrates to the world the height of that state of Divine love. After one hundred years, Krishna and Radha met in Kurukshetra during a festival which was attended by all the Yadus.