I presume you have already checked all the connections.
Switch off pc completely (wall plug) disconnect the Hard drive, AND all other, printer etc.
switch pc back on and see if bios boots and displays on screen.
(to switch off the pc after just hold the power button on for about 5-10 seconds until it switches off)
If it does then repeat the above with HDD connected
and repeat for each piece of 'extras' cards, printers etc. adding them one by one until you find the one that is causing problem.
It could be the graphic card if you have one fitted (not onboard)