https://www.youtube.com/watch?v=GoFJ_0fLf3s
I've had exactly the same - this would be for the second time. I bought these in early April, then had the power on issue and had to replace them at the end of April and now the problem has occurred again. I've noticed that when they fail to work the power indicator flashes 3 times in white then goes off. I've tried the volume button down + power reset procedure but it doesn't work.
To get around the issue I've noticed that if I wiggle the neck band where it connects to the battery that houses the power button (eventually) the headphones will start working again. On the flip side I can wiggle the neck band after and the headphones will fail to power on again.
I've tried getting them replaced with apple over their support but they are asking for my headphones to be sent in which really isn't convenient for me as I these are my only pair of headphones and I frequently use them for studying. I don't even own a pair of wired headphones any more and given the frequency of this issue I would expect it to be handled better than the current approach offered by Apple.