What do you mean when a man suddenly doesn't answer his phone or return his text messages?

The first possibility, as some netizens said, is that he is too busy to pay attention to you.

The second possibility, as some netizens said, is that he doesn't want to talk to you.

The third possibility is that he may be tired of you and don't want to reply to your text messages or answer your phone calls.

The fourth possibility is that he may not have received your phone calls and text messages, so he will not return your text messages or answer your phone calls.

The fifth possibility is that he already has someone he likes. He is afraid that the person he likes will misunderstand him, so he won't answer your phone or return your text messages.

Sixth possibility, you may have offended him unintentionally, so he doesn't want to answer your phone or return your text messages.

Seventh possibility, you may have inadvertently ignored him.