I have bought 2 in the last 3 months and both of them will not turn back on after I turned them off. I have tried to reset them like I read on here with no results.
Have you charged them?
Have you connected them to a computer?
Are you sure you are pushing the power button?
They are under warranty for a year (if they are new) but it’s unlikely you would have two with exactly the same problem. Check the manual to make sure you are not missing some step.