If the player cannot be turned on after pressing the Power button, the first thing to do is to check if the unit is running out of power or if the charger you use fits the requirement (DC output,  voltage: 5V, current: 0.5A or above).
If it cannot be turned on after being charged for some time or even fully charged, try the following methods (They also work for the problem of freezing).

For the X1, X3, X5 & X3 2nd gen:
a) Reset the unit by poking the Reset hole(beside the Power button or at the top side of the unit) with an unfolded paper clip or other slender tools.
b) Connect the unit to the computer via USB then reset it as method a. (method b is not suitable if the player is dead after charged with a wrong charger)
c) Try a force shut-down by Pressing and holding the Power button for 15 seconds(for the X1, it's poking the Reset hole and holding for 6 seconds), then try to turn it on.

Since the X1 2nd gen,X5 2nd gen, X7 & M3 don't have a Reset hole, the only way to try is to force shut-down by holding the power button for around 15 seconds then turn it on again.

First, please check if the M3 is sufficiently charged. You may try to connect it to a Power supply for around 10 mininutes or longer then turn it on.
If it still cannot be turned on after that, try to hold the Power button for at leaset 10 seconds to force shut it down, then turn it on again.

Please check the method above first. 

If the issue still remains, please inform us when did you buy the product? And which country are you located in?