It could be a dead motherboard or burned out cpu, but I always test the power supply first.
A power supply has three power rails, 12,5,3.3 volts. A power supply can lose one two or three rails and create odd results.
In your case, the 12 volt (runs anything with a motor) and 5 volt (power switch and lights) are working -- but the 3.3 (video, ram, motherboard, cpu) could be gone. Typically, a fried CPU and mainboard means that NOTHING happens at all on pressing the switch--this is not 100 percent, though, depending on what died on the mobo
This results in a machine that seems to be on, but is not.
Easy check. Remove a Power supply from a known good machine (need adequate wattage--at least 300). In bad machine, unplug the power connectors running to the motherboard from the power supply--you can leave the power supply in there. (one or two connections) You do not need the drives connected for test.
Take the good power supply, connect to the motherboard while it is still outside the case. Put it on a book so it is not touching anything metal to avoid short. Hit the power switch. If the PC boots--it will not start windows without drives, replace the power supply. You should get video and bios hardware detection
When I turn on my computer , the monitor says that it is in "power save mode "----now, I know that it is not the monitor because I can connect another computer to it and it works fine I cant even get into the bios on this computer Everything else seems to come on except video monitor It has no built in video on the motherboard so the video card is my only option. How do I get the computer out of this mode ? I've also tried a different monitor and same thing happens. Whats up ? Is this a motherboard problem or is it a power problem? 
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