Soil

If its true soil you need to refortify it and ideally run it though a clean period where it gets really hot in a garbage bag. If its soiless you can rinse it out and rebuffer if its core and go from there. It could also benefit from a hot resting period to cook anything out of it and encourage good bacteria to flourish. Obviously like others have said below, if the medium was contaminated by something chemical "bleach" or biological "pests/fungas/virus exct." it would not be good to reuse it till its cooked. The trick is noticing that before you reuse it. If growing outdoors I just mix everything into my existing soil bed and grow in it, the wilderness takes care of it mostly. Good Luck!