On Clio's fifth birthday, reached the age where she was deemed ready and able to ascend the throne. However, she was poisoned by her Uncle Ferdand, who was acting on behalf of his son Malvek, ruler of Dolmara. The First Doctor, who was present for her birthday party, contrived a machine that would keep her in stasis, preventing her from dying.
It was the Doctor's intention that Clio be woken on her birthday every year, but because war had broken out between her father and Malvek, it was not always possible to revive her every year. The Doctor visited her in each incarnation, bringing her presents.
Clio was unaware of the passage of time while she was asleep, and wondered why her friends and family members seemed to be ageing. She was not told that her planet was at war.
Finally, there was no power left to keep her in stasis. The Eighth Doctor visited her at this time, when all her friends and family had died. He took her outside for fresh air before she succumbed to the poison. (PROSE: The Glass Princess)