It probably needs more light. As long as the branches on the bare side are not rotting, they will regrow.
Ficus are very sensitive to changes in light. If you go from a high light location to a lower light, they will drop every leaf. If you go from low light to outdoors the leaves will burn.
Most bonsai are outdoor plants and that is where they should be spending most of their time. Gradually harden off the tree to go outside. Indoors try to give it more light. If you have a shelf or counter where you can mount and overhead flourescent light it would be better. The plant should be inches away from the light source. If the light is more even the tree will not lean as it is doing now and the leaves will be more evenly dispersed.
Try not to over water the plant and do not let it sit in water. Bonsai outdoors need daily watering, in the house it usually needs less. Also water the plant in the sink let it drain well before putting it back on the tray. The tray should not have any water in it. Pebbles in the tray help keep the plant up higher in case there is some water and help with humidity.