I'm a complete novice to bonsai. However, my sister gave me a bonsai as a birthday present in late July; it's a Serrisa Foetida and I'd say that it would have been about a year old when she got it. The place where she bought it is a general nursery with a small section devoted to just bonsai.
Unfortunately, it seems to have died for no apparent reason. I'd followed the instructions it came with pretty much to the letter and when it started showing signs that it was under stress I tried everything to revive it. After the leaves had dropped off, I cut one of the branches to see the color of the cambium - very faded green, almost grey. The moss around the base of the trunk had also died.
Just as I was about to throw the whole thing out, I noticed (pic 2) that there was a tiny shoot beside the now dead tree coming up from the soil. It's been growing now for about two weeks. I'm not sure what to do though - I'm worried that whatever killed in the first place will eventually kill the sapling. I'm tempted to just pull the dead tree out and cut it off from the new part, or even just start from scratch with a cutting from a different tree.
I'd be grateful for any advice you could give me.
Here are the pics

