When a desktop won't turn on, the first step is to check the power supply. I would start by ensuring that the power cable is securely connected to both the desktop and the power outlet. If the connection is secure but there's still no power, I would try using a different power cable or outlet to rule out any issues with the current setup. Next, I would inspect the power supply unit by checking if the power supply fan is running or if there's any indication of power, like a light on the motherboard. If there's no sign of power, the power supply might be faulty and may need replacement. If the power supply seems fine, I would then check the motherboard for any visible damage or loose connections. I would also listen for any beep codes that could indicate specific hardware issues. If the motherboard appears to be receiving power but the system still won't start, I would proceed to reseat or replace the RAM and check other components like the CPU and GPU to ensure they are properly connected. Throughout the process, I would be careful to handle all components with care to avoid static damage.