The message usually appears when the UEFI/BIOS firmware cannot locate a valid boot loader. If the drive that contains the operating system is not listed first (or at all) in the firmware boot order, the firmware will attempt to start from another device and fail, producing the error. Setting the correct drive first in the boot sequence-or enabling the drive if it is disabled-typically resolves the problem. Corrupt video drivers, incorrect display settings, or minor RAM-speed mismatches occur after the OS begins loading and therefore would not generate an "Operating system not found" error.
References: Lifewire lists a misconfigured BIOS (wrong boot order) and a damaged or unreachable hard drive as leading causes of the "Operating System not found" error; SalvageData likewise cites an incorrect BIOS boot order as a common cause. [Lifewire - "Causes of the 'Operating System Not Found' Error: BIOS misconfigured, hard drive damaged"] [SalvageData - "Check BIOS Boot Order" section].
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