I have a 400mhz computer where I have changed the power supply and the computer does not power on at all and the hard drive power cable was pulled when it was on. I am completely lost on this one.

If I understand correctly, you have a 400 Mhz computer and the power was unplugged while it was running. It will no longer post. You have tried to replace the power supply and it still will not post. Is that correct?
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yes the power to the hardrive was unplugged while it was running

Did you unplug the power to the HD or did it fall off by itself?

And have you put the powercable back in?

the power fell out which doesn't make sense but when I opened the case it was unplugged and I did put it back in and that didn't help

Check on all other HD-cables, both on HD and motherboard, make sure they are solidly in place.

If you can, perhaps use another powercable from PSU to HD. Where there "burn"marks on the HD-connector, after the powercable fell off? It may be that the electronics on your HD are fried.

if im not mistaken the computer should try and do something even if there is no hard drive connected (no power) so that means its either the motherboard or the power supply.

could you post some information about the rest of your computer? especially what motherboard and power supply.

all i can suggest is to try another power supply with everything disconnected except the motherboard and videocard. you should be able to observe something happening. :grinthumb

Last time I saw someone pull the power cable from a CDROM and/or HDD, they blew the whole system. It surged through all the components due to the spark that was created and very little was salvagable. I agree with strmn8tr to test the PSU, but I would test all the devices individually if you have the opportunity. Good luck

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