What color is the powerled? Amber, blinking amber, green, blinking green?

Also, what do the diagnostic leds on the rear show? Especially if the powerled is green these diagnostic leds may provide a hint as to what's going on.

A general thing you could try (you may have already done this, but thought I'd mention it), is to disconnect all cables from the unit, press and hold the powerbutton for a few seconds, then plug in the power cable only and try to power it up. If you get a green led, you might be getting somewhere... add a monitor and try again and see if you get something on the screen.

Info on the powerled color (front): here

Info on the diagnostic leds (rear): here

strip the computer down to just the psu, cpu and motherboard as the computer should still turn on with just those components. That includes removing all cables from the drives and any cards/memory connected onto the motherboard. If you have the ability to test another CPU in the system try that too, or if not just reseat the CPU. If its still solid amber with another CPU then you need a new motherboard and if you dont have another CPU to test with you would have to replace both the CPU and motherboard to be sure..or you could make a best guess and just replace the motherboard as it would be more likely

It did turn out to be the motherboard. Swapped it out and back up and running. Thanks for the posts.