Sounds like a processor problem.

 

The Processor could have gone out explaining the system hang, and it will not boot because it has no valid processor.

The computer will POST even if the hard drive and RAM were not installed. or not working.

 

So In my expertise, I say Processor problem,

Just order the same processor frrom dell by going HERE and entering your service tag to get the correct processor for your system and then order a new processor.

Or what i suggest, taking down all the specifications and ordering it from a third party company, tends to be cheaper then Dell.

Also if you want to upgrade the processor, then make sure it is compatible with the motherboard, and that you have enough RAM to support it. I also suggest getting a memory upgrade because a 16 month old computer will have a ok amount, You should have about 1Gig,