If you turn on a microwave that isn't plugged in, absolutely nothing will happen. The microwave requires electrical power to function, and without being plugged into a power source, it won't receive any electricity to operate.
Modern microwave ovens are designed with safety features that prevent them from turning on or functioning if they are not properly connected to an electrical outlet. When you press the microwave's power button or set a cooking time, the device won't respond or display any signs of operation if it lacks a power supply.
As a general rule, never attempt to use any electrical appliance, including a microwave, if it's not properly plugged in, as doing so could pose safety risks and potentially damage the appliance. Always ensure that your microwave and other electrical devices are correctly connected to a functioning power source before attempting to use them.