you boot up, and the computer cannot find the drive C
If you have another computer, swap hard drives to diagnose the problem. If your computer boots with the other drive, yours is probably bad. Sometimes, a reboot will work. Your drive might have enough life to spin up occasionally. If this works, transfer your data to another drive, pronto. According to techie lore, you can seal a nonworking drive in a bag and put it in a freezer overnight. That could shrink things enough to free them up. I've used this trick a few times and it's worth a try.
A regular backup regimen will save you in case of hard-drive failure, assuming you aren't backing up to the same hard drive. If the drive is dead and you don't have a backup, a computer shop may be able to save your data. Hard drives are cheap. You can get a replacement for less than 0.
Get one boxed for retail, which will include instructions and any hardware needed. Your hard drive is in the front of your machine. It will be about the size of a paperback book and is probably held in by four screws, two on each side. Power and ribbon cables connect to the back. Put the new drive in and install it as the master. Reconfigure the old drive as the slave.
The instructions that come with the new drive should explain that. Boot the computer and install Windows on the new drive. If you're lucky, the computer will see the old drive (it will be D:). You can then transfer your data to the new drive. Replacing a hard drive is more difficult than the other operations. However, if you pay to have the work done, it may not be cost effective. You might be better off buying a new machine. So if you are adventuresome, and you have the time, changing the hard drive may be worthwhile.