Lancelot had emerged victorious! Now he could bring Guinevere safely back to Arthur's court, to the great increase of his fame and reputation (and perhaps the queen's regard for him) ! But alas, she would not even speak to him now! Why ever not

The answer is that Lancelot hesitated before he enter the cart.

Explanation:

According to Chretien de Troyes story, Queen Guinevere was kidnapped by a knight after a challenge gone wrong. Lancelot got some clues to her whereabouts and went to rescue her, which he did after successfully evading some dangers.

However he hesitated to enter the cart which was supposed to take him in search of his lady because such carts were usually used to display criminals on their way to punishment. He knew it will certainly bring him dishonor, and so he hesitated to mark himself in that way.

Queen Guinevere got angry at Lancelot because he had hesitated to sacrifice his reputation and honor, which are dearer to any self-respecting knight than life itself, for his love for her.

The queen forgave him however, and she explained his offence to her when he begged her to reveal he reason for her anger so that he could avoid annoying her in the future.

He acknowledged that he his behaved rather badly, and was grateful for the correction.