If your Gateway 400Sd4 Ac Adapter is not powering your laptop, here are some simple steps you can do before you get a new AC adapter

1) Check the AC Adapter

Take your battery and ac adapter out and then reseat the Battery and the AC Adapter. Check for any damaged connections or connectors.

2) Update the BIOS (Basic Input-Output System )
A computer’s Basic Input-Output System (BIOS) is embedded software on a motherboard that will control attached hardware. It provides an operating system with information about hardware, and is designed to support a specific range of components. An out of date BIOS can cause Unknown AC Adapter issues. Visit the the your laptop manufacturer’s website to for latest BIOS updates specific to your computer.

The 2 steps above are not limited to Gateway Ac Adapter only. After you have done the above and your power cord is still now working, it is time for you to get a new one. It costs you an arm and leg to get the OEM AC adapters ( Original Equipment Manufacturer, for example,if your laptop is DELL, a DELL AC adapter is OEM adapter). Aftermarket AC adapters are perfect solution to this. They usually cost only 1/2 or 1/3 of the OEM ones and they work the same.
You might notice that PowerUP AC adaptors have a higher mAh rating than the original one supplied with your laptop, this will NOT affect your laptop in any way. It is very important that the voltage is the same however so please do check this before using your new laptop ac adaptor.