Hi, I’m really new to the bonsai scene and a complete beginner with plants in general. I’ve had this bonsai (I don’t know what species) for maybe a few months but recently I’ve noticed that it lost at least half of it’s leaves in the span of a week. I don’t think I’m overwatering nor under watering it, and it’s directly next to my window so it isn’t a lack of light either. It honestly could be those things since, as I said before, I’m pretty new to this, but I’m really kind of stumped on what’s going on here. There are also these fuzzy brown patches where the leaves once were which might be normal but as I don’t know the species of the tree I’m not really sure if it’s normal for it. Also sorry for the picture quality, I took these at night.

The species is fukien tea tree (carmona). What is happening is quite normal for fukiens growing in windowsill. Come authum, your heating system kicks in and dry the air up in your house. Also the tree gets less and less sun. That is unfavorable conditions for the tree and it suffers. Eventually it will die. Proboably before spring,

We have hundreds of questions on dieing carmona. Nobody comes back and show a success story. The tree was sent to a certain death when it left the production facility.

I too am very new to bonsai. However quite experienced with plants. So let me take a stab at this. My Fukien Tea made it through last winter with supplemental plant light and increased humidity. If it's cold outside where you live, bring the plant away from cold windowsill. Try using a bypass pruners to cut away the tip of you tree branches to see if there is still life in there. If so, leave it be and it will re-foliate. If the branch looks dead (dry, brown color) keep clipping little by little till you see life (greenish). Have you reported it since you bought it? Maybe a bonsai club can show you . Also, there are some excellent videos on YouTube.