2008/9 Mac Pro wont turn on, front pannel LED is constant white My mac pro doesn't want to start up.  

I purchased some new memory installed it (and cleaned up some of the dust on the inside of the box) and now the machine won't boot up.  I get a constant white LED on the front, no fans, no chimes, no other lights light up (RAM riser boards)?

I have removed the new RAM and replaced it with the old, in an attempt to just get the machine to boot.  No luck.  

So now the box is back in its OEM state, but still wont boot.  I've tried the 15sec unplug / hold the power button down for 5 sec rest and  that didn't seem to work.

Any idea what might be going on?

I also swapped out a known working RAM riser from another box and still can't get it to boot up?
Sadly, the most likely cause is that you zapped your system board. If you used a household vacuum cleaner to remove the dust, this is highly likely.