Now that turn of phrase catches Strange's attention. Fixed everything they had done? Well that logically means that they did something in the first place. Strange pauses for a moment, frowning as he thinks, and connects all the dots. A few moments later, the penny drops.

Oh. If he's right, and he hopes he's not right, then that would explain why Childermass is so tired.

As such, he's deathly serious when he asks, "What do you mean 'fixed everything they had done'?" He doubts he'll get an actual answer. After all, Childermass has a bit too much stubbornness to admit that he might have been seriously hurt. But it's certainly worth an ask.