The only time I felt I wasn't breathing was the very first time I smoked DMT. I thankfully remembered hearing that users often reported the sensation of not being able to breathe, so it didn't scare me and I just kinda ignored the fact that my body wasn't doing what it usually seems like it needs to do. Some days after that happened, I read some article that despite the fact that users sometimes indicate not being able to breathe, when their breathing was monitored, it didn't show any fluctuations, so the user only perceived to not be able to breathe. They continued to theorize that it was due to massive time distortion from the rush of incoming data so that the time between breaths can seem to take forever. I remain a little skeptical of this explanation, but I haven't really had much of the chance to observe/test this theory as I haven't had troubles with breathing since then. If it should happen again, just remind yourself that nothing bad will happen and that if you redirect your attention to something else, before you know it, you'll be involuntarily breathing again.
"Science without religion is lame. Religion without science is blind" - Albert Einstein
"The Mighty One appears, the horizon shines. Atum appears on the smell of his censing, the Sunshine- god has risen in the sky, the Mansion of the pyramidion is in joy and all its inmates are assembled, a voice calls out within the shrine, shouting reverberates around the Netherworld." - Egyptian Book of the Dead
"Man fears time, but time fears the Pyramids" - 9th century Arab proverb