Summarize the following:
If the Prius fails to start when you hit the power button, try applying more pressure to the brake pedal. The brake pedal sensor must register that you are pressing it in order for the computer to allow the electrical motor to start. Be sure the floor mat hasn't gotten bunched up beneath the pedal. Check for any other obstructions that may prevent you from pushing the brake pedal down. If the battery in the smart key is beginning to die, you may be able to start the car by holding it closer to the power button on the dashboard. The sensor for the key is located near the ignition, so this will reduce the distance the signal must travel. If this method works, you should have the battery replaced in your smart key as soon as possible. Some models of Prius come equipped with a smart key deactivation switch. If this switch has been pressed, the sensor will not read your smart key and will not allow the vehicle to start. Check in the owner's manual for your specific model of Prius to see if it came equipped with a smart key deactivation switch. Locate the smart key deactivation switch on the underside of the dashboard beneath the steering wheel. Press the switch and then try starting the vehicle again. If the smart key deactivation switch was pressed before, hitting it again will reactivate the smart key sensor. Attempt to start the car again once you've hit the switch.

Summary:
Ensure your foot is pressed firmly on the brake. Hold the smart key closer to the power button. Determine if your vehicle has a smart key deactivation switch. Press the deactivation switch beneath the steering wheel.