Hello,
15 years ago I bought a bonsai from a shop.
I was just giving it shape to its branches but i never actually repotted it.
I left for vacations and the person that was supposed to care my my plants never watered it so it lost almost all its leaves.
I started searching (for the first time) on info about bonsai and discovered i should be repotting the tree.
When i try to repot it i discovered a the mess. It almost has no roots !!!
Please i need advice on what to do to save it.

 

I can't see what species it is, or if it has any leaves, but I do know you should not have re potted a tree that is as stressed as that one was. The fact that it was not watered for some time means that it is probably dead. Dead does not come back to life. You have I hope put it into a pot. You can water and hope but I doubt it will survive. 

could be a Ficus, but not sure

It did survive. It just lost a lot of leaves.
I need advice on what to do to the tree to promote root growth 

There is nothing much at this time of year, just watering. Good light for the rest of the year. Then when and if it starts growing it will need fertiliser, but not ow.