My yahoo id and password hijacked by some one

My yahoo id and password automaticaly stollen by some one. when I logon my offline messages just flickering and lost. When I Look my message archive one particular id (it’s not in my chater)named sweetguy_2022 appear.
It is seen me that “Magic-PS v1.5 SE++ ===> ” and my user id and password.How can I stop that?