After Zara pissed off Shigaraki in the USJ attack (knocking off the hand from his face was something he holds grudges over), he gave Dabi the bonus objective to kill Zara in the Training Camp attack (on top of Capturing Bakugo.) Zara managed to not die, but she suffered from smoke inhalation and was burned by his ultra-hot flames that melted her crystal at about 2000 Celsius. She lost consciousness for a while and so Shoto had to carry her.
Incidentally, Dabi’s flames burned enough of her clothes that her abuse scars/bruises/injuries are visible. However, no one can really distinguish them from the burns, so no one asks her about them. It takes Zara some time to fully recover, especially emotionally, after the attack. The smoke inhalation + inhaling some of the intense flames burned her mouth and throat a bit so she can’t talk at first. Until Recovery Girl works her magic, that is, and then she takes a long nap.