|Yeah, I think you'd be getting some kind of error message is the problem was associated with the hard drive.|
Does it give you enough time to get into cmos/bios setup?
It's possible but not likely it's booting into windows but there's a video setting the monitor can't handle. As soon as you turn it on start tapping F8. If the boot menu comes up then go into safe mode and change the display properties to something basic, maybe 800 x 600, save and reboot.
If you don't get the boot menu then the problem is likely with the motherboard, power supply or both. If you can, swap in a known working compatible power supply and see if that makes a difference.
While you're inside the case, as Trek suggests, check that all connections are tight, check that any added cards are firmly installed, remove and reseat the memory and check that the cpu heatsink is properly attached.
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