My Computer won't boot

Hello,
My neighbor brought her computer over so that I could check out if it was working out properly. Before she brought it over, it would power up without any problems and boot into the operating system (Windows XP). When I plugged it into the power supply it initially booted up but I had to turn it off abruptly (before WIndows loaded) since someone needed to take something beneath the computer table. When I tried to power it up again it wouldn't boot up. Now when I try to turn on the computer, the cd rom drive light gets turned on but it seems the monitor can't read anything from the hard drive and the monitor becomes blank. I tried going into the bios but the monitor can't read from the bios either. I think I may have a problem with my motherboard but I'm not sure. I desperately need help since the cost of fixing this computer will ultimately rest on my shoulders since the computer malfunctioned at my place. Can someone please advice me on how I can troubleshoot this problem and ultimately fix it. Please help! I also forgot to mention that when I opened the the system box cover I found that the hard drive had not been inserted into it's position and it was just hanging loosely and one of the screws that keeps the hard drive in place was lying inside the box cover. I'm in desperation please help.