Can not get computer to power back up no lights on front o rear just little light on mother board have dell 8400

Bad Power Supply, sscorbett.

Usually you have enough power to light LED lights, (Like the harddrive activity LED light for add-in cards, that you see on the motherboard), and spin fans, but Not enough power to turn the Processor on.

Dell Support >Dimension 8400 > Service Manual

http://support.dell.com/support/edocs/systems/dim8400/SM/index.htm

Technical Overview > System Board Components

http://support.dell.com/support/edocs/systems/dim8400/SM/techov.htm#wp1052739

harddrive activity light for add-in cards (SCSI LED)

The Dimension 8400 comes with an ATX style of Power Supply, that has a maximum rated Wattage of 250 Watts.
1) ALL of the LED lights combined use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts
3) A typical Processor can use 51 to 125 Watts.

Your Power Supply just has enough power to light that LED light.
Weak voltage power rail.
Replace the Power Supply.

[ There are three power rails inside that SMPS.
Switched-Mode Power Supply.

1) The 3.3 Volt power rail
2) The 5 Volt power rail
3) The 12 Volt power rail ]

The ATX form factor for a Power Supply refers to the size, and shape of the case, for one.
Approximately 6 inches Wide, 5.5 inches Long, and 3-1/2 inches Tall.
The ATX power supply case in the Dimension 8400 however, is shorter in length.
It is 4 inches Long. (Depth)

Not an advertisement for the website, nor manufacturer.
Reference for the case size, and power cables needed,

http://www.cputopia.com/hp-400w-dell-dimension-8400.html

Scroll down to see the connectors of the various power cables needed.

You may have a Power Supply with these dimensions, and needed power cables, in an unused computer.
Perhaps someone has upgraded to a better computer, and has an unused computer that you can temporarily borrow the Power Supply for a test unit.

Make sure it has at least 250 Watts (Maximum rated wattage), plus,

1) 20-pin ATX main power cable. (Or a 20 + 4-pin ATX main power cable)

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20

[ 20 + 4-pin ATX main power cable,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20plus4 ]

2) 4-pin ATX +12 Volt power cable

http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4

Plus the various power cables needed for the Harddrive, optical drive/s, and computer case fan/s, if it/they do not connect to the motherboard.