First, check the basics — power cable, outlet, and battery connection.
For desktops, ensure the PSU switch is on and the motherboard light is glowing.
Hold the power button for 10 seconds to discharge static, then try again.
If nothing happens, remove external devices (USBs, monitors) and boot bare.
For laptops, reseat the RAM and battery if removable.
If fans spin but the screen stays black, it could be a display or GPU issue — connect an external monitor to test.