I had exactly the same issue and it turned out to be a faulty hard drive. At first I could boot, say, 7 out of ten times, then it got less and eventually the "operating system not found" message appears all the time as the hard drive breathed it's last. It all happened within the space of a week. Scandisk didn't report any errors, even after a full surface scan.
Take an up to date image as quickly as you can before the disk fails so you are covered - in case this is the same problem you are experiencing.