Ingratitude is the quality of not being grateful — of taking help, kindness, or a gift and then acting as if you owed nothing, or even turning against the person who helped you. It is a rather formal, moral word. Parents, teachers, and old stories use it; younger speakers more often call a person ungrateful, and use that adjective far more than this noun. You will meet phrases like an act of ingratitude. Though actually, it is heavier than “forgot to say thank you.” It means a failure in a duty of thanks, and someone is hurt or angry about that.

忘恩負義