John Gill Commentary 2 Samuel 3:11

"And he could not answer Abner another word, because he feared him." — 2 Samuel 3:11 (ASV)

And he could not answer Abner a word again
That is, he would not, he dared not.

Otherwise, if it was fact he charged him with, he could have insisted on the truth of it, and aggravated the crime and scandal of it; and observed it to him, that the kindness he had shown him was no excuse for it; but such things, though he would, he dared not say:

because he feared him ;
he had the army at his command, and could dethrone him when he pleased; and it has been the fate of greater men than Ishbosheth to be awed by their generals, and even David himself; see (2 Samuel 3:39) .