If your screen is lit, but there is no writing on it (no letters or numbers appear) and you can still use the phone to make calls, you may have accidentally lowered the LCD contrast. The display will not disappear completely in this case. You can try tilting the phone a bit and see if you can make out what the display should be. If you can see the display, go into the menu to LCD Contrast and darken the contrast.
You may also have the wrong AC Adapter plugged into the phone. This would cause the phone to have enough power to light, but not enough power to operate all functions of the phone properly. Unplug the AC Adapter from the phone (not while speaking on it) and read the information that appears on the label on the adapter below the prongs. The output should be 9 Volt 200 mAh. If the voltage and amperage on the adapter you have are different, please call us to see if we can get the correct adapter to you.
If your screen is not lit at all, check and make sure the AC adapter is plugged into an active electrical outlet. If it is plugged in, try it in a different outlet and see if that changes anything. You can also check the power on the adapter to make sure it is the correct one. (see above)