Reactivate a closed Yahoo account https://help.yahoo.com/kb/SLN2624.html
That's unfortunate. Why are you still calling this person your friend? Friends don't hack friends' personal accounts and mess around with them or close them.
you need to contact yahoo about it and i would turn your friend in for doing that
This is how to recover your account if you have password/sign in problems: https://help.yahoo.com/kb/SLN2824.html