Henry uses Yahoo Mail, and he got a message from Yahoo forcing him to change his password. Leo says that Yahoo gets hacked a lot and they may have noted some activity on Henry's email and prompted him to change it. But now he can't access his account. Leo says that's a good signal that his account had been hacked. He probably won't have much luck contacting Yahoo to fix it, either.
This is why it's important to have second factor authentication enabled with his email, so that if anyone tries to change his password, he'll get a text message to confirm it. Yahoo has terrible security and it's time to abandon it. He should go with GMail instead.