This is one of the worst kind of error messages you can receive. Typically, however, one can conclude that either the hard drive is not functioning properly, the BIOS cannot detect the hard drive — or — if you’re really lucky, it’s a simple, dumb mistake like leaving a USB flash drive in the computer and the BIOS is attempting to boot from it or the wrong HDD has the priority for boot. That can easily cause the operating system not found error message.
The system that we’re talking about here is an old Dell Dimension from the Windows98 era. This particular computer had been used by various people over the years as a gaming,, Internet and mostly email station. It had never been opened before, so it surely had tons of dust from the 90’s in there.
Because the system has functioned without any hardware failures over the past decade, I immediately assumed the hard drive had finally come to it’s natural end and crashed once and for all.