Momc,

Probably what's happened is the momentary contact switch behind the power button has slipped out of its mounting.  The 8400 service manual is here:  support.dell.com/.../index.htm 

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While the service manual can be of great assistance when removing and replacing parts, it's not much help with your specific problem.  The manual does give the location of the motherboard connector for the front panel switch.  

To get to the front panel switch, you will have to remove the front panel.  Instructions for the 8400 are similar to those for the 8300 which is shown here: <ADMIN NOTE: Broken link has been removed from this post by Dell>

Once you remove the front panel, you should be able to access the switch board located behind it.  Remember to be gentle in your attempts to remove and replace the panel.  With the front panel removed, you should be able to reconnect AC power and see if the switch is working.  Also, inspect the back of the front panel button to be sure it isn't broken.

Always remove power to the system when opening the case.

Hope the information helps.

Note that you do not have to create a new post to comment on your previous post.  Simply click on the Reply button to add additional comments.  In my opinion, installing Windows 7 has nothing to do with your problem.

Tony

After upgrade to WIN7 ultimate  when computer was not in use it would seem to go into sleep mode. Frist let me say I upgraded on Weds.3/28/12. After that I noticed that when computer was idle for a while it would go into what I thought was sleep mode, I would push power button and computer woud power up. This morning(Sat, 3/30/12) when pushing power button the button would not power on. The  lgreen light on  the power button is not evident. I don't thin it's in the power supply because when you plug it in there are lights on the mother board. I can see where the button is supposed to connect but am not sure how to get to it.It appears the button on the front panel is not connecting to the red button thats benind it. How do I get to it? Is there a way to fix this?

The power switch and circuit board is one unit. Dell has not published a guide to remove the front bezel of the Dimension 8400, you can use this GUIDE,  

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it's for an Optiplex GX50 Small Mini Tower, the procedures are the same.

A hint: The Dimension 8400 also, has a Hex headed screw on inside right side of front bezel/panel, just below the front USB ports, holding the front panel to the metal chassis, this MUST be removed and is not mentioned in the guide.

Bev.

Nothing runs at all.

momc

All the Dell Dimension 8400 motherboards have a Socket LGA 775 for the processor.

You can install any version of the Dimension 8400 motherboard, the only difference is the chipset, the earlier motherboards have the Intel 925X Express chipset and later versions of the motherboard have an intel 925XE Express chipset. 

Bev.