Hi
I suspect that it has something to do with the power saving or sleep modes. Can you get the machine to run now with a display ?
If not try clearing the CMOS by:
Remove the power cable from the mains supply and then press the power switch on the case for a few seconds just to drain any residual energy in the PSU capacitors.
Once done remove the motherboard battery for at least one hour before replacing it.
Next plug back into the mains supply and boot.
You will now need to enter the BIOS by pressing DEL and load Optimised BIOS Defaults.
Make any other changes to the BIOS settings to suit your self like disabling the floppy drive, disabling the full screen logo and making the HDD the primary boot device and then press F10 to save and exit.
Once done and hopefully with the system running again go into the BIOS and disable all the power saving and sleep modes that you can find. Save and exit F10.