Dimension 8300 won't start.

Hey Guys- thanks in advance for reading.

My 8300, after running flawlessly for years, froze right after bootup and login.  No BSOD, and I couldn't CTRL+ALT+DEL out- I had to manually power off.

After that the power light blinked amber and it wouldn't turn on.

I blithely figured it was the power supply, and promptly replaced it with a more robust one. (I was kind of excited, I hadn't done a computer repair in *years*)

Now the power button still blinks amber, the green light on the lower right part of the motherboard is lit, but still no power up, no fans, no drives, nothing.

The four diagnostic lights on the back are not lit either. 

What do you think it could be?

Also, is there anyway to tell if the motherboard or CPU are still good?