How to Power Cycle a computer that won’t turn on

Problem

Your computer will not turn on. There may be no sounds, lights or fan activity when you press the power button.

Solution

Select your type of computer below:

I am using a Desktop running Windows

1. Turn the switch off the back of the back of the Desktop.
2. Turn the switch off where the computer is plugged in to.
3. Hold down the power button on the front of the computer for 10 seconds.
4. Turn the switch on at the plug.
5. Turn the switch on at the back of the computer.
6. Press the Power button on the front.

I am using a Laptop running Windows

1. Unplug the charger.
2. Take out the battery.
3. Reconnect the charger.
4. Press the Power button.

I am using a Mac

Apple have their own set of detailed instructions here:
Resetting the System Management Controller (SMC)