Let's start with some of the basics.
1) Since you were checking connections on the motherboard, was the power unplugged from the computer, AND did you follow Anti-Static precautions?
Your body carries Static electricity.
Static WILL fry out (Short Circuit), the hardware components inside a computer.
You may not even see it, or feel it.
Computer unplugged from power, computer case open, computer on a table.
TOUCH the metal frame of the open computer case to relieve your body of Static.
Should you get up in the middle of working on your computer, and leave, upon your return Touch the metal frame again.
Or use an ESD wrist strap, and connect the alligator clip to the metal frame ]
2) Computer unplugged from power, anti-static precautions followed, remove all ram memory modules, and reinsert them. In fact, it would behoove you to clean the contact pins on the bottom of the ram memory modules, before reinserting them.
Clean with a pencil eraser. Remove the eraser dust, with air pressure from a can of compressed air for computers, or air pressure from your mouth may be sufficient.
You may have bumped the ram memory modules.
Even if they look to be seated, it only takes a few minutes to remove, and reinstall.
After time, corrosion can develop on the gold plated contact pins, at the bottom of the ram memory modules. Doesn't take long to clean them with a pencil eraser.
Handle the ram memory module by the body. The body, is coated with a clear see-through protective plastic.http://en.wikipedia.org/wiki/File:Generic_DDR_Memory_%28Xytram%29.jpg
The above link, is only provided for an illustration of the gold plated contact pins, I am referring to.
It is DDR Sdram ram memory, and has 184 contact pins. 92 on each side.
Your computer uses DDR3 Sdram ram memory.
Has 240 contact pins. 120 on each side, and the Locating Notch is in a different place.
3) Are you sure the 4-pin ATX +12 Volt power cable, is plugged in tightly to the motherboard?http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4
This is power for your Processor.http://support.gateway.com/s/desktop/2009/gateway/dx/dx4820/DX4820nv.shtml
SATA power cable is plugged in tightly to your SATA harddrive?http://www.playtool.com/pages/psuconnectors/connectors.html#sata
If your SATA harddrive has provisions for plugging in a SATA power cable, and a 4-pin Peripheral power cable, BE SURE that only the SATA power cable is plugged in.
Plugging both in will fry out the harddrive. Maybe not right away, but will down the road.http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral
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