Hello! There’s several reasons why your phone might not be turning on:
1: Battery is dead. Sometimes the battery is so dead, it doesn’t even show a “no battery” warning when you try to turn the phone on. Try charging it, and if a little charging icon appears just give it some time and it should be good.
2: Power button fail. Occasionally, the button can fail, and this would require opening up the phone to fix.
3: Board is dead. This is arguably the worst option. When the phone is turning on, it does a few hardware checks (Like a computer) where it checks to make sure it has RAM, a CPU, etc.. Unfortunately this is very difficult to fix, and would require the motherboard’s schematics and knowledge about microsoldering.