What did Ahithophel do when Absalom rejected his advice?

Answer: Went home and hanged himself

Seeing his counsel was not followed, Ahithophel saddled his donkey, went home, put his affairs in order, and hanged himself.

“When Ahithophel saw that his advice had not been followed, he saddled his donkey and set out for his house in his hometown. He put his affairs in order and hanged himself. So he died and was buried in his father’s tomb.”— 2 Samuel 17:23 (BSB)