I am a third-year Ph.D. candidate. Recently, my Ph.D. advisor suddenly started ignoring me. He only talks to me whenever he needs my help in our lab's work. Except that he neither reply my email nor Teams message. He even ignores my request for a recommendation letter or reviewing a finished paper manuscript. What should I do? Should I look for a chance to talk to him or complain to the department?
DO NOT complain about him. It leads nowhere good. Have a sit-down. There might be something going on in his life. He might, actually, feel that you need his help less and he is putting limited time/effort elsewhere. And he might have a misunderstanding generated by who-knows-what. A sit-down is the only way to resolve it.
Complaining will put you in a worse spot almost certainly.
If you have other, trusted, members of the faculty you can talk to, they might be able to tell you whether the professor is having personal difficulties. Other students might be able to tell you whether they detect a change of behavior.