Make sure the hard drive is actually connected to the motherboard and that it is connected to the power supply. If it is move on to the next step.
Insert your windows 7 disk and boot to it. Select the option to repair an installation. It will search for your installation. If it finds it, start the repair. After it finishes, take the disk out and try to boot. If it still wont boot, move on to the next step.
Buy a HDD docking device and connect it to a working computer. Put your HDD in the dock and copy all of your important stuff to the working computer. Buy a new HDD, install it, install Windows, and have fun.