Sounds like either the hard drive is going, or the motherboard is having issues. Maybe the hard drive was poorly connected to begin with, and a very small "thud" was sufficient to break the connection? You could open it up and see if that's the case. For a hard drive, it might not void the warranty to just open the correct back panel and take it out, and make sure it can connect, etc.

But beyond that, this is likely to be a hardware issue.