OS not found.

When your computer displays the message "Operating system not found," it usually indicates a problem during the boot process. This issue can arise from several causes, including incorrect BIOS settings, a damaged hard drive, or issues with the operating system itself.

One common cause is an improperly configured BIOS. If your BIOS is set to look for the operating system on a non-existent or disconnected drive, it won't find the OS and will display this error. To resolve this, enter the BIOS setup (usually by pressing F2, F10, or Delete during startup) and ensure that the primary hard drive is correctly identified and set as the first boot device.

A damaged hard drive can also lead to this problem. If the drive where the operating system is installed is faulty, the computer won't be able to load the OS. You can try running diagnostic tools provided by the hard drive manufacturer or use utilities like CHKDSK in Windows to check for and repair file system errors.

In some cases, the issue might be related to the operating system's boot sector. This can happen if there's been a power failure during the boot process or if malware has corrupted critical files. Using a bootable recovery disk, such as a Windows installation media, you can attempt to repair the boot configuration using commands like "bootrec /fixmbr" and "bootrec /fixboot."