What happened to Radha in her later life ?
Romance of Radha and Krishna
Radha-Krishna romance has been an epitome of love for God and a subject of uncountable paintings, stories and poems. Radha's impact on Indian psyche is so deep that even modern bollywood songs will feature her.
However we hear very little of Radha in Mahabharata and Puranas once Krishna has moved out of Mathura. Radha's mention in Bramhavaivrata Puran and Geet Govind is most prominent of all texts.
Puranic records of Radha's whereabouts
As per this text Krishna is the supreme being and lives in Go-Loka. In his avatar on earth he tells Radha that they have to be separated for 100 years however he meets her in her dreams every day. After Krishna's avatar on earth is over he returns to Radha and they spend a lot of time together in Vrindavana. Later they together move to Go-Loka forever.
Geet Govindam was written by great poet Jaydev in 12th century. His songs laid the foundation of Radha Krishna romances from which we continue to get inspired even today though Jaydev did not add about Radha's eventual fate.
Some of the folk tales mention that Radha had married to some King later but could never be happy in that marriage and eventually turned ascetic. Historically there is no unanimity on Radha's actual existence. According to some Radha is that perfect consort and a secret of Krishna that is beyond human comprehension. It is a celestial love story and we do not truly know what Radha represents.
According to some Radha was a real person in Krishna's life thought their relationship might have been made popular by later poets and may not be very accurate reading of what might have actually happened between them.
Conclusion
No matter what we believe I think the stories of Hindu mythology have no start nor any end. Whatever might have happened to Radha there is little doubt that her pure love and devotion carved a space for her in hearts of millions of people generation after generation.