My keyboard is not working at all

If you had to remove the keyboard to get to the hard drive, it's likely you need to reseat the keyboard's ribbon cable. Please be careful, as the retaining clips are fragile and are both easily broken and impossible to fix. However, if you did not remove the keyboard, it's quite possible the keyboard is bad. It's not very common (it's usually only a partial failure), but it can happen. I'd check the connection either way, to be sure. In extremely rare cases, it could be the motherboard itself, but I've only ever encountered that once, and it was due to the connector's retaining clip having broken.

Unfortunately, I couldn't locate a service manual easily... Dell usually offers those freely, but it doesn't appear to be available. I'd check with them anyway, just in case. I might have missed something.

If replacement is required, it doesn't appear that Dell offers this one, currently. You'll have to go through a third party. I like eBay for the built-in protections, but BE CAREFUL. (I only deal with Top Rated Plus sellers with a high "feedback score" and over 95% positive feedback, as well as a good return/warranty policy.)

Also, keep in mind that you can use an external USB keyboard in the meantime, if you have to order a replacement.

Good luck. I hope this helps!