Windows won’t start

'Operating system not found' error at startup

The screen shows 'Operating system not found' or 'No operating system was found' instead of Windows starting. The BIOS can see a drive but can't locate a valid, bootable copy of Windows on it.

Difficulty
Intermediate
Time
20–60 minutes
DIY cost
$0–$60
Pro cost
$100–$250

Symptoms

  • Error text mentions operating system not found
  • Appears right after the manufacturer or BIOS splash screen
  • PC otherwise powers on and fans run normally
  • May follow a BIOS update, drive swap, or power loss during use

Likely causes

  • CommonBoot order set to the wrong driveEspecially common after adding a second drive or USB device.
  • CommonCorrupted boot sector or partition tableDamage here prevents the BIOS from recognizing Windows even though files may still exist.
  • PossibleDrive cable or connection issueA partially connected SATA or power cable can make a healthy drive appear invisible.
  • PossibleDrive failureA dying drive can pass basic detection but fail to present a bootable partition.
  • PossibleBIOS/UEFI setting mismatch (Legacy vs UEFI mode)A drive installed in one mode won't boot if the BIOS is set to the other mode.

What to do

  1. Check the BIOS boot order and confirm the correct drive is listed first; reorder if a USB or second drive is ahead of it.
  2. For desktops, power off and reseat the SATA and power cables on the boot drive.
  3. Check the BIOS/UEFI boot mode setting; if Windows was installed in UEFI mode, Legacy/CSM mode won't find it, and vice versa.
  4. Boot from a Windows installation USB and use 'Repair your computer' > Command Prompt to run 'bootrec /fixmbr', 'bootrec /fixboot', and 'bootrec /rebuildbcd'.
  5. If those commands can't find any Windows installations, the partition table may be damaged and may need diskpart or third-party partition tools to inspect.
  6. Confirm in BIOS that the drive itself shows up in storage information at all; if it doesn't, the drive likely needs testing outside the PC.
  7. If the drive isn't recognized in BIOS on a different cable/port, treat it as a hardware failure and prioritize data recovery.

Tools and parts

  • Bootable Windows installation USB drive
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When to call a pro

If the drive isn't detected by the BIOS on any cable or port, have a data recovery specialist check it before attempting any repair yourself.

Safety

Generally low risk

Generally low risk for a careful person. Stop if anything looks different from what this guide describes.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.