You said " the only sound is the fan.
There are two 1 in power supply
1 on processor.
When you switch on does the screen runthough a lot data[this called P.O.S.T] then give a beep?
If it does not then the machine has mother board or memory fail problem.
When you checked it over were you proected again anti static discharge?
Open case again.
Take hold a metal part of chassis to balance your electric potential
and chech th MEMORY card it tight inits slot.
Then all the connections.