Operating System Not Found error

When I start my computer, I get this message saying "operating system not found."
However, I can still reach the BIOS settings. So I check on it; then load the default settings; save it; then restart my computer. This helps it start normally. But every time I turn it off, I have to go through the same process again and again. The error keeps reappearing every time I turn my computer off and back.
