Possible reasons your device "Wont power on"
1) Hardware issues:
a) Screen: It is very possible that your device is able to power on, but the screen is not working. Charge the device for approximately 15 minutes, then go through the routine of powering up the device. If you get LEDs, or Sounds, then the screen isnt working. Wait 5 minutes, then plug the device into the computer, if it starts to install driver software, or you get a signal through the PC, then it is a Screen issue. ---- If you get a Battery Charging image on your device, then the screen is fine, and you can move on to the next step.
b) Charge Port: You can have issues with the Charge port on the device, not accepting a signal. Plugging in your device should give you some signal that it is charging. (Noise, LED, Screen display) if you get any of these, then your charge port is fine, and you can move onto the next step.
c) Battery: you could have a cold (Always reading zero%) or a shorted, (Always reading 100%) on the battery. plug in the device. if it reads a mid range level, and % goes up, your battery is fine, and you can move onto the next step. However, if the % starts at 100%, and the image of the % goes away immediately after removing the charge port, the battery is bad.
d) Power button: your device may not boot due to a cold circuit in the Power Button. If the power button is not sending a signal, it will not tell the board to boot. Use a multimeter in Diode mode, on the connective pins of the power button, and check for continuity. the Multimeter should beep as you press the button. If it does, it is funtional. if it doesnt, you have a cold.
d ii) If your multimeter checks positive for the continuity, (Makes beepy noise) then, you may have a connection issue, where it is not signalling from the button, to the board. in which case, you would need to check the connection. (make sure all pins are meeting their traces without obstruction.
e) board level failure: Your board is the nervous system of your device. it does all of the thinking. so even if someone presses a button, your board needs to process it. Your board could be failing to process the function you are looking for. in order to fix this issue, you will likely need to acquire Board Schematics, and data sheets, and use a multimeter to compare acquired voltages to expected voltages. this requires a specific skillset in SMT (Surface Mount Technologies) Soldering. Replacing the board at this point is the easiest option.