Hi,
have you taken the heatsink and fan off the CPU ?
If so - you need to clean both surfaces of the old thermal paste and apply new. ( please ask if unsure )
I assume that you get no POST beeps ?
Take a look here:
click here
This will walk you through the checking process.
Follow it carefully :)
This could be HEAT as well.
Check inside the case for dust - especially the fans and vents.
Try running the system with the side panel off.
Do you get any further with the boot process now ?