Pic?

Create a humidity dome for the plant. If it is a largish plant easiest is to find an appropriately sized clear plastic storage totes. Or use a plastic sheet with supports to enclose the plant. You still want it to breathe fresh air a bit otherwise it will be too humid and encourage fungal growth. Water the plant well and keep it just moist there after.

Thanks for the suggestion. I had already put a plastic bag around it but found something better. I’m going to try to post a photo of the poor thing.

From the picture looks like a lot of the top branches may be dead. Did you try snapping or cutting to see if there is any green under the bark. Also there are way too many branches for a bonsai. I suggest you to clean up all the dead branches (usually they snap easily if they are indeed dead). If it has not been repotted in a while that could also have contributed to the situation. I bet there is a dense mat of roots at the bottom and sides of the pot that are mostly dead. You can check by lifting the plant out of the pot. Essentially, there is no soil mass left for new healthy roots to grow into. A tropical would need to be repotted every two-three years to ensure root health. If so, then you should prune off all the dead roots from the side and bottom and pot the rest in a bonsai soil and use a slightly bigger pot if possible. Water well and let it be in the humidity tent for a while.

OK, I just took the bonsai out of its pot and removed a lot of tiny branches. The good news is there‘s green everywhere, even inside the teeniest branches. I’m now trying to remove most of the old soil before repotting it. No need to put it in a slightly larger pot because the roots were not circling each other or even as far as the edges of the pot. I probably had it in too large a pot to begin with. I’ll take a few more pictures. Appreciate your suggestions, tropicofcancer.

Another comment, I like a full, bushy style of growth which is why it had so many branches. I’ll trim it back nonetheless.

Use a proper bonsai soil. Although expensive, "bonsai jack" has good premixed stuff. Best time to do a full repot is June. So I suggest keep the inner ball of soil intact (hopefully the existing soil is also a proper bonsai soil. Otherwise try to match up the existing soil and do a full repot in June. Assuming you are in Northern hemisphere and in a cool/cold climate zone. If you are in a warm/hot zone you can do a repot earlier.

Also you can achieve bushy look with less branches. You need good light penetration and air flow otherwise you will end up random branches dying.

I usually have pruned the top canopy fairly hard at the beginning of summer. I know about a bird being able to fly through a bonsai’s branches. LOL. Yes, I had a good quality bonsai soil on hand, I was wondering how to deal with that inner soil ball, so thanks for addressing that. Yes, I’m in a cool weather zone. As for your last sentence, I don’t think I’ve ever had any branches die off. Like I said, beginner’s luck probably.

In June I would not hesitate to clear off the old soil in the root ball completely. It takes some patience and some practice too. Depending on the variety of serissa they can exhibit the behavior you noticed. It could have been that it dried out too much. But too much water can be equally good cuplrit. I have variegated pink flowering one that seems to be least finicky. The japonica likes more to be on dry side in winter. The kyoto likes to be evenly moist . Keeping temperatures a bit on the lower side (55-65F) seems to be helpful to get through the winter.

Wow, I never see the types of serissa you mention! No problem keeping temperatures down in VT! I’ll pat myself on the back if my serissa lives through the ordeal it just went through. I grow orchids too, but sometimes I like bonsai better.