I was working on my Inspiron 9400 while it was charging when suddenly the screen flashed and then the computer went off. Since then it wont come on. No lights or screen glow. Its dead.
Can anyone please help?

does the rest of the machine still do something or is everything non reactive?

Its a piece of plastic. And is dead as a door nail.

what you can try first is to unplug anything unpluggable:

power supply, battery, pcmcia cards, usb devices, firewire devices, anything attached to the laptop
if you can also remove the memory bars.

once done hold the power on button for like 30 seconds. dell calls that a hardware reset (at least they told us here).
done that just plug in the power supply again, try to switch it on and see if something happens.

I took the laptop apart last night and put back together again. After which I tried this 30sec thing. But it still wont work.
I've had this laptop for less then 3 years and is in very good condition.
Is it possible that a power surge caused it?

that is possible, yes
not sure if I can help you more with that. do you still have warranty for it?
if not you can trick them a little by buying a warranty extension for 1 year (appr. $40) then wait 1 week and call them for a repair. they will replace all broken parts "for free".

I don't like doing that kind of thing.
But if it is a power surge then what needs to be replaced?
I can then buy the parts on ebay

if it was a power surge then most likely the onboard power converter thingy is broken. in many case its more than that so you probably need to replace the board. I would let a technician have a look at it first. these things are hard to determine remotely

replace the power pack and/or batteries and rertry.