There is no such thing as an indoor tree. Not one tree in the entire world lives naturally indoors.
Of course there are (sub)tropical species that die in our not tropical climate and need protection to survive our colder periods. But there comes the problem: indoor is read and understood as: in your living rooms, which is completely lethal to most, if not all trees. Our living rooms are waaaay to dry and lack (sun)light. Indoor should be read and understood as: protect from cold(er) temperatures by placing it in a not heated room untill teps outside are high enough again to go back outside.
As Tropfrog says, there are hundreds of people that came here with the same question, and/or the same dead tree ( chinese elms, fukien tea), all socalled indoor trees.
i have never in all the years that i am on this forum seen one person return to show how good their tree is doing indoors.
So, your question is what to do first: nothing, absolutely nothing except water it and place it in a relatively cool room for the winter, or if temps in you part of the world are still suitable for the Fukien ( lets say above 10-15) you can still keep it outside but keep a good eye on frostnights