When a desktop won't turn on, the first step I take is to check the power source. I ensure that the power cable is securely connected to both the desktop and the power outlet. If the desktop is connected to a power strip or surge protector, I verify that it is switched on and functioning properly. Next, I inspect the power supply unit to see if the fan is running, indicating that it's receiving power. If there's still no response, I might try using a different power cable or outlet to rule out a faulty connection. Additionally, I check for any visible damage or loose connections inside the case, such as the motherboard or RAM. If the desktop still doesn't power on, I consider testing with a different power supply unit to determine if the original one is faulty. Throughout this process, I ensure that I'm grounded to prevent static damage to the components.