Why is my 3ds not turning on?

Could you elaborate a bit more? Does it show a red light, or a blue light, does it have custom firmware ect.

It would be good if you could post some more information about what is going on with your DS. For example when you try to turn it on do any of the LEDs such as the power light turn on? Does the charging LED turn on when it's plugged in? Are you running some custom software on your DS? Do the screens remain completely black or do they

If you plug your DS on and none of the LED lights turn on that could mean that your charger is bad and you need to try charging it with a different charger.

If your charger isn't the issue and the lights still don't turn on with a good charger plugged in that could mean that you need a new battery.