I've had experience with a slot style proc. that was loose in the slot and wouldn't power up until it was seated good, so I assume if the proc is dead or broken the odds are just as good that it wouldn't power up.
If your floppy cable is connected wrong (assume your talking about ide cable) the light on floppy usually just stays on but it will still come on.
The last comp I worked on had a trashed video card and it powered up but never completely posted. It will check your ram and find your drives but will never display video or complete the post.
To do a basic test just plug in the following parts
PSU, System board Speaker, Cable for power button and CPU with Heatsink/Fan.
If you power up you will get a memory error beep, if not then it's down to PSU, Sysboard, or CPU. some mobo's will still power on if there is no CPU but you will get nothing apart from the PSU fan spinning.
Only the insane prosper.
Only those who prosper can judge what is sane.