Kirsti asks: "My son took his laptop apart because the fan wasn't working. He blew dust out of the fan, and it's all better now, but now the keyboard doesn't work. How can he fix it?"
As Kirsti's son found out, it's generally easy to take a laptop apart and clean out the insides, given a few screwdrivers of varying sizes. I have done this many times with my own, because I have several cats and the laptop gets clogged up with cat hair over time. I just pulled apart a friend's laptop to replace the DVD drive, as well. It's just held together with a lot of screws--we had to remove 16 screws to get the bottom off of my friend's laptop.
The problem with digging around inside a laptop (besides the fact that you're likely to void your warranty by doing so) is that it's very tight inside there, and there a lot of connections in a very small space. I am sure Kirsti's son didn't break the keyboard: Most likely he just didn't reconnect it correctly. There is generally a very small connector on a ribbon cable that attaches the laptop keyboard to the motherboard of the laptop. If you don't attach that just exactly right, the laptop keyboard won't work. This is easy to test: If the computer boots up but the keyboard doesn't work, try connecting an external keyboard. If that works, most likely the existing built-in keyboard just isn't connected right. (Of course, it is possible to break a laptop keyboard--pour a can of Coke on it, for example, even when the computer's off, and that keyboard will most likely never work again. But that's a completely different topic.)
If, on the other hand the keyboard is really broken, it's generally easy to replace the keyboard. You can often find replacement parts at the manufacturer's website. I have also found Amazon.com and Ebay to be useful places to find individual parts for my laptop. There are a lot of "fly-by-night" computer parts dealers online, so find a reputable dealer, and you should be able to replace any part, including the keyboard.