The main thing you need to worry about is watering it correctly. Doing so incorrectly is the fastest way to kill a tree

if it is gone due to lack or otherwise of water, then nothing you can do now except learn from it.
if it is disease or infection of some sort then maybe

Have a look at the wiki here for info on how to water correctly for your own environment and hope for the best

https://www.ausbonsai.com.au/wiki/index. ... w_to_water
ps Welcome to Ausbonsai

Ken