Trying a different wall port to plug into. There might be an issue with the current line they are plugged into.
There are two components to the power cable (the cord that runs to the computer connected with the power brick, and the cable that connects to the power brick to the wall outlet). Make sure all cables are secured and not easy to pull apart.
Hold down the power button for 20 seconds. Observe any changes to the device such as the screen lighting up or and of the LED lights lighting up or flickering.