Is there any sound or sign of power at all? Make sure it is connected to a power supply. Try holding the power button for 20 seconds in case it is already on (this will force it to power down if it is on but non-responsive) but just non-responsive. Then release and try to power on again normally.
Also worth trying: boot in safe mode (hold SHIFT key when starting up), and boot into recovery (CMD-R) from which you can run Disk Utilities to check your disk or elect to reinstall the OS (without erasing or losing any data).
Try resetting the SMC: https://support.apple.com/en-us/HT201295
Try resetting the NVRAM: https://support.apple.com/en-us/HT204063
If still non-responsive (black screen, no sign of power), the next step I think would be taking it to an Apple Store or an Apple-authorized repair center.
Good luck!