I left my Dell Inspiron 1420 on overnight and when I checked it the next morning it was completely dead. It was operating on AC power, without the battery in.
The AC adapter light was out when it was attached to the laptop. When I disconnected the adapter from the laptop and switched on the mains, the light came on, and it like it could function normally. However, once I connect it to the laptop, the adapter light goes out.
The laptop cannot power on on battery alone, or with battery and AC power connected either. The battery does not charge either.
The laptop gives no response (no lights no sound) when the power button is pushed. Even without any peripherals attached, the laptop cannot power on at all.
I had not installed any new hardware devices or software, except for the fact that I do leave my laptop on overnight quite often. I do not think it could be due to overheating as the laptop temperature has never gone above 'uncomfortable to touch' levels of heat, and I always use a notebook cooler.
What could have caused this problem?

Unless it's the power adaptor that's actually faulty?

Try removing the power adaptor and battery
Hold the power ON button on for 30 secs (dispersing all internal voltage)
Plug the power adaptor back in, and then try turning it on.

Did it work?

no..it did not work...the moment i plug the adapter into my computer it sort of goes out...and there's absolutely no response from the laptop whatsoever

Hmm

Ok try the above, and then just plug in the battery (assuming the battery has some charge)

i've tried that too..and my laptop still doesnt give any sign of life...

Always check the adapter socket. They cause this problem if the soldered connection goes bad. But it is usually the adapter that fails.

Time for a trip to eBay. Get the information from the back of your current adapter, then buy a new one that is an Official Dell unit. Too many of the Hong Kong and Singapore imitations emit spurious responses that affect radio, wireless, and some TV connections.
All adapters go out eventually. Yours is unfortunate, but not unusual. Feel lucky it did not damage your laptop. (or hope it didn't)

sorry if i sound completely clueless..but which part is the adapter socket?

That is the hole into which your power adapter from the wall to the brick to your computer plugs. It is the interface between the laptop and the power adapter. You might call it the "recepticle". It is the power hole on the back or side of the laptop.

so what am i supposed to be checking for in my adapter socket?

You need somebody with a volt-ohm or multimeter to check the connection from inside the case.
The usual failure is a loose connection. If it wobbles, or feels loose under light pressure from a wooden probe, it could be bad. Sometimes due to the pressure of constant use, it can break loose from its "flat surface" soldered connection.
First test, really, is the output of your power adapter... or buy a new one, Dell branded, on eBay.
Replacing the socket is not for beginners. It requires a special soldering iron, usuall a "cold solder", that can perform soldering on the flat surfaces. You need to know how to solder two flat surfaces together without ruining something else. Not many techs can do it at all, let alone well. Anything less than a first class job by a a very skilled technician can make it impossible to fix at all.
Check other things first, then consider sending it to Dell.

ok...so i called dell tech support...as i am based in kuala lumpur, malaysia, i don't know if this applies to other countries...but i find dell tech support here excellent...i called in on monday morning and told them about my problem and on tuesday morning a technician shows up with a spanking new motherboard and replaced it on the spot...my computer works fine now....

You're in Dell's home town.
But yes, under warranty, they will do the same anywhere in the world, for free.