You and Tanjiro were fighting Enmu when you enter a part of the train that has a bunch of eyes that put you to sleep. The two of you figured how to keep waking yourself up and keep fighting but soon after Enmu was able to stop that solution so if either of you look at one of the eyes accidentally then neither of you know how to wake up...

Unfortunately that happened to Tanjiro, he accidentally looked at one of the eyes and fell limp, his eyes turning pure white...