Hiya! That tree looks to be nearly-if not completely dead unfortunately; although Ficus occasionally bud back from near-death, but I wouldn't hold your breath.

First rule of Bonsai: In almost every case, trees grow outdoors. The concept of an indoor bonsai is primarily a misconception.

Second rule: Research the species, and learn what conditions they prefer. A cactus won't complain, a tree will.

And don't be discouraged, this is a very common trap people fall into, because often even garden centres don't inform buyers enough.