A hard drive is a part of your computer assembly. IN the old days people used to call your Motherboard, CPU, Memory, Video card, and hard drive the "Hard Drive" so it's hard from your description to tell what is broken.
If your computer displays things on the screen but it says "hard disk failure", then all you need is a new hard drive, they can be relatively cheap. Seagate and Western Digitial are good brands of hard disk. YOu just unscrew the case and put the new hard drive in, then re-intall windows or have someone else do it.
If your computer doesn't even turn on, no sournds or anything, your entire computer could be fine, and you could just need a new power supply. Those are also cheap and easy to install.
But if you lost your motherboard or CPU, then you need to buy pretty much a whole new kit. Dell makes some nice desktops.