Originally posted by Jason S
If you are talking about the pin header for the power button/LED, the missing pin might be normal. Check the pin block, it should be solid where that pin is missing. As for the computer still not starting does it have diagnostic lights on the rear? Most desktop Dells have a A-B-C-D led readout giving diagnostic status readouts. Dell.com openly tells you the codes...good luck.
This is true for newer Dell ATX motherboards. The 4100 is a mATX and doesn't have this diagnostic feature.
As far as one of the pins being snapped off inside the the plug... That's not a good thing. Is there enough of a piece of pin left in the connecter header? If so try bridging two of the wires with a screw driver. That's probably one of the pins for for the Power switch.
If all else fails... call Dell and see what they suggest?
Possible a new motherboard, if they even exist anymore...