I moved my Ficus inside in early October before we had any "cold" nights. I put it in a corner where it gets indirect light and it stays warm but it has lost almost all of its leaves since I brought it in.
Why Is My Ficus Tree Losing All Its Green Leaves?
Certified GKH Gardening Expert
Ficus do not like to be moved and will show you by dropping it's leaves.
Check for any signs of insects since the plant was outdoors.
Neem Oil is a good treatment if you find any issues.
Make sure the plant is getting adequate light and the proper watering.
Here is a link with more information.