Whilst this is a fairly common topic name, I'm pretty sure its a less than common cause.
A week or so ago I had my setup my PC running on XBMC playing videos, only to find the next day it had frozen. When I restarted I had a DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER error. My immediate thought was "oh no, my hard drive has died" but that is when I realised that neither hard drives were being detected. I tested and the drives were picked up in my other PC, leading me to conclude that it was beyond my skills and a good excuse to buy myself a new mini PC (Acer RL70, loving it!).
Yesterday I decided not to let my old PC become an oversized paperweight and even if it wont detect my SATA hard drives, it'll still pick up USB ones so I installed Ubuntu, spent ages faffing about to get onto my network share and I was away again. This evening I started watching my favourite video playlist and again it froze, possibly the same video. Rebooted and low & behold... now my internal and external hard drives aren't being detected with the same boot error. My external was detected by my Windows laptop so I don't think its the drive, although it never mounted (I'm assuming because its got Ubuntu on it?).
Short version: My PC froze and upon reboot I got a DISK BOOT ERROR with no SATA hard drives detecting. The same happened again when I was booting from a USB drive.
Has anyone seen anything like this before? My motherboard is an Asus A8N-E btw. Technically I bought the PC, although I think the only original component from when I bought it is the DVD drive (which still works!) so I hope I'm okay in this board.