Your computer starts when you turn on the "Power" button--at least, that's how it's supposed to work. If absolutely nothing happens, then your computer is not starting and you have a serious problem that can range from a bad power supply to a loose connection of hardware connected to the motherboard. If your computer starts and says it cannot find the hard drive, your computer has technically already started, but to access it as normal you are going to have to do some troubleshooting and repair.
Check your hard drive's connection to the motherboard, especially if you have recently changed anything. If you have more than one hard drive, disconnect the other drives and only connect the hard drive with the operating system. If the hard drive is an IDE drive, make sure the jumper is set to master and that it is connected on the farthest-out connection on the IDE cable.
Restart your computer and see if you have the same message. If your computer still gives you a message that your hard drive is not found, then you will need to replace the hard drive. Computer hard drives fail more than many people realize, which is why backing up is so important. If your drive has failed, it is too late to back up. Occasionally pulling a hard drive out of a computer and freezing it will temporarily give it more life--hopefully long enough to back up anything you haven't already saved elsewhere.
Buy and install a new hard drive. You will have to install Windows or another operating system on the computer from scratch. Put the Windows install CD in the CD drive and restart the computer with the new drive in place. Follow the on-screen instructions to install Windows.