1) Side Cover off; Observe whether the fan that pushed air over the Processor is operating.
2) Remove all ram memory modules, and reseat them.
Doesn't take much to bump a ram memory module ('Stick') loose.
Although they may appear to be correctly seated in the their individual ram slots, it is just added assurance to remove, and replace them.
3) Computer was unplugged from power, and if you reached inside, you did observe Anti-Static Precautions, correct?
Anti-Static Precautions:
Your body carries Static electricity.
Static will fry out (Short Circuit), the delicate hardware components inside a computer.
You must relieve your body of Static BEFORE, reaching inside your computer.
Computer unplugged from power, Touch the metal frame of the open computer case, to relieve your body of Static.
As also has been stated, make sure all cables all plugged in tightly.
This is Sony eSupport > Vaio PCV-RX650 desktop computer >
http://esupport.sony.com/US/perl/model-home.pl?mdl=PCV-RX650®ion_id=1Left-click on Manuals, Specs, Warranty
Under the Manuals heading, left-click on System Reference Manual