If you turn on a microwave that isn't plugged in, absolutely nothing will happen. A microwave oven requires an electrical power source to function, and if it's not plugged into a working electrical outlet, it won't receive any power. Consequently, the microwave will not turn on, heat, or operate in any way.
It's essential always to ensure that electrical appliances, including microwaves, are properly plugged into a grounded electrical outlet before attempting to use them. Additionally, it's important to follow safety guidelines and instructions provided by the manufacturer to avoid any accidents or damage to the appliance.