Looks like an elm of some kind. I am unsure of the specific variety, but I will say that Zelkova serrata (white bark Chinese elm) is a popular bonsai species. I understand that some elms will behave as an evergreen indoors, but generally they will drop leaves in response to shortened daylight hours (I keep mine outdoors and they seem to drop leaves primarily because of the first frost). Based on the health of the tree depicted in your photo, I would expect it to start leafing out again in a few weeks time.
Meanwhile, you can scratch the bark with your thumbnail (all the way down to the wood inside). If you see a line of green around the wound, your tree is alive. You can also watch the buds that are located where the leaves used to attach to the stem and soon enjoy them getting larger, then 'cracking', and watch your new leaves emerge!
Also, meanwhile, be careful not to overwater. The soil should feel dry (at and near the surface) before you water. Roots must get oxygen; overwatering will literally drown the roots and kill your tree. Of course the roots must remain moist, but the most common beginner mistake is to overdo it.
