Your Gateway should have the power cable plugged-in in one of those ways, either on the side of the Gateway or directly to the core. It is either a barrel plug cable or a Mini USB cable. It can be plugged in like any of the below four pictures.
Step 1: To reset/restart the Gateway, unplug the power cable (barrel cable or mini USB cable), either from the gateway or from the electric socket (whatever is easier for you).
Step 2: Wait 10 seconds and plug it back on. The lights should turn back on.