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Boot up the computer that is having an operating system not found an error and then insert the DVD/USB. Then press any key also from the drive. When keyboard, language, and different needed settings, and press Next. Then choose to Repair your computer. On the select, an options screen, click Troubleshoot. Select advanced options and open command prompt. Open command prompt
In the command prompt window, type the following commands pressing Enter after every one of them: bootrec.exe /fixmbr bootrec.exe /fixboot bootrec.exe /rebuildbcd Finally, restart your computer.