|Have you actually seen that computer running in the shop, or is it just what the staff have told you?|
Make sure you are getting power at the power socket you are plugging that computer in to. If it's getting power, try this:
Turn off the computer's PSU with the rocker switch at the back.
Press and hold the front power button for 15 seconds.
Release the power button.
Turn the PSU rocker-switch back to the "On" position.
Now press the front power button to start up the PC.