So if you disconnect/take out the hard drive, are you saying you don't even get the Post screen or anything showing on the monitor?
Is the monitor giving you the 'no signal' message/orange light (on standby)?
Do you have onboard graphics or a separate card? (and have you tried both) it could be graphics card failure if you get nothing at all on monitor screen.
Any troubleshooting should be done with hard drive disconnected, until you can successfully get a Post screen and can view Bios