you should approsch him and try to find out why he has stopped
talking to you
absolutely not . in worst case scenario, if you may have done something to offend him/her, leave a note saying apologies briefly and to the point. However when you have done nothing wrong and they aren't talking, avoid them. They are no longer interested .
maybe he has something on his mind. Try asking him if there is anything wrong. You don't need to avoid him. If he just wants some alone time, give it to him, after a while you can go back and talk to him.