I am so sorry to hear about your mother's tragic accident. It has to be hard to pick up the pieces after that. May she rest in peace. Give time to ease your grief.

I see life in that stem so don't despair. At least it is a cane which is the toughest of all begonias to kill. If the bottom showed signs of dead wood then I'd suggest cut off the top showing life and water root it.

First of all, do give it a drink and then let the soil dry somewhat before watering again. Second of all, give it as much light as possible. This does not mean putting in full sun. Give it a bright location that gets filtered sun outdoors or if you keep it indoors then move it close to a sunny window. Fresh soil might be a good idea but I don't see any need to do that right away (maybe after it puts out new growth). Once it bushes out once again then you can think about repotting, adding slow release fertilizer, etc.