Hi and welcome to Android Forums!
Not knowing which device you have, I have this general answer for you.
To turn on your Android Device, long-press the power button until you feel a vibration. It will boot up.
If the device is not responding to the power-up procedure, remove the battery (if it is removable). reinsert the battery and power up with the long-press. If your battery is non-removable, you might want to refer to the manufacturer for additional help.
I hope this helps.
It’s great to have you here