In case you did not find an answer yet, there is a link below that I found regarding health conditions and proper care for your tree.
"Unhappy serissa bonsai trees shed their leaves," so this technically means your tree isn't completely unhappy, if it hasn't let go of its leaves yet. Keep it out of DIRECT sunlight, and not too much water. Having it indoors by a window might also be a good idea, so that it can recover in a stabilized environment (despite it being a good outdoor tree). Hope this helps.

Here is a link that might be useful: Serissa Bonsai Trees

its not shedding its leaves though, they are just wilting and hanging on. It also seems like the bark has cracked and the insides look white- are they supposed to be green??

Pretty sure your tree died when left to its own devices outside (I don't know if you're brave or just very optimistic, but serissa is not a tree I'd play Russian roulette with like that). If you can get another one, do whatever you were doing prior to going away and if necessary, leave it with a bonsai nursery when gone, or at least someone you really trust to follow explicit directions.