I'm assuming this is F. benjamina, the tree with small leaves. They will grow perfectly happily with zero sun, even in very dark positions like a north-facing window. What they don't like is change of any kind. I would expect a tree moved from outside in the sun(?) to inside may well drop most of its leaves. It will then grow new leaves adapted to the new position. It might drop them again when it goes outside! Don't overwater it in winter. Maybe a good time to repot since it is going to drop its leaves anyway :)

Time will make your ficus glad again. They generally don't appreciate being moved, especially into less light than they are used to. Check and make sure that it's not getting cold drafts from anywhere (windows at night, a door that opens to outside frequently, leaky window or door, etc.). That will cause yellowing as well. So will watering habits that the ficus doesn't like.

The good news is that it will grow more leaves again when it goes outside next spring.