What sounds do you get? Any fan or drive noises?
Do you get any `beeps` when you put power on?
What exactly have you done with the hard drive? This tells you exactly how to remove/fit a HDD to a desk top PC click here
If the HDD has failed it may have caused secondary damage, but it may also be the power supply unit (PSU) that has gone down. You could also be right about the motherboard, but all avenues should be considered.