By Lucas / Last Updated October 22, 2021

User story

I installed Windows 10 on an Acer computer about four days ago and it worked quite well. Then I put some things on my computer. I thought it was running slow so I reset it (delete everything) from settings. While it was deleting (2%), I shut it down (accidentally). I powered it on then It said "Operating system wasn't found. Why? What do I do to fix this problem? Thanks a lot!

Operating system  Not Found

Why is the computer saying no operating system found?

Have you ever met the operating system not found or missing operating system error on your Windows 10/11, especially the HP, Sony, Acer, Lenovo computer? It would prevent you from booting Windows successfully. Before attempting to fix this error, first, you should figure out why the message occurs.  There are some reasons for operating systems missing or not found in Windows 10/11:

●The system hard disk is damaged.

●The basic input/output system (BIOS) does not detect Windows 10/11 Installation hard drive/disk.

●Windows 10/11 boot files in the system partition are not active anymore.

●The MBR (Master Boot Record) of Windows 11/10 is corrupted.

4 Methods to fix Operating System not found in Windows 10/11

According to the based possible reasons, here we rounded up 4 methods to help you fix the no operating system found a problem in Windows 11,10, 8, 7. Now, lets get started:

Method 1. Test hard disk problems

First of all, you can test the hard drive for problems. To do this, you can take the following steps:

1. Turn off the computer and wait for 5 seconds.

2. Press the Power button to start the computer and repeatedly press the F10 key to enter the BIOS Setup menu.

3. Use the Right Arrow or Left Arrow keys to navigate to the “Primary Hard Drive Self Test” option. Depending on your BIOS, this may be found below Diagnostics or Tools.

4. Select Primary Hard Drive Self Test, and then press the Enter key to start the test.

The Hard Drive Self Test will perform a Quick Test followed by a Comprehensive Test. During each test, the window displays a status bar and an estimated time to complete.

>>If all tests passed, the hard drive is not damaged. 

>>If the test failed, you should contact your hard disk or computer manufacturer for a replacement as long as these devices are under warranty.

Method 2. Check the BIOS Settings

If the system hard disk cannot be recognized by Windows 11/10, the Operating System not found  error will pop out and interrupt the Windows 11/10 startup process. Check the BIOS settings to make sure the BIOS whether detects the hard disk or not, then determine if the hard disk is damaged.

1. Press the F10/F12 key while turning on the Windows 11/10 computer and release F10/F12 when the BIOS SETUP UTILITY shows up.

2. Check the Primary IDE Master, Primary IDE Slave, Secondary IDE Master, and Secondary IDE Master to see if the hard disk is detected.

If displayed as "None" or "Not Detected", that is to say, the BIOS does not detect the hard disk, you could change "None/Not Detected" to "Auto". Maybe it solves this problem.

Restart your computer to test your problem. If the problem is still there, you should try other solutions or replace it.

Method 3. Active Windows partition

When the partition on which the Windows is installed is disabled, the Operating System not found issue may also occur on Windows 11/10. In this case, you can fix it by reactivating the Windows partition.

1. Turn on your machine and boot it from a Windows installation media. In the Install now” screen, click on Repair your computer”.

Repair Your Computer

2. Click on “Troubleshoot” > “Advanced Options” > “Command Prompt”.

Access Win 10

3. In Command Prompt window, type the following commands and press “Enter” after each one:

●diskpart

●list disk: A list of all the disks attached to your machine will be displayed. Find out the system disk.

●select disk [number]: replace [number] with the system disk number.

●list volume: it will show you all partitions on your selected disk.

●select volume [number]: replace [number] with the system partition number.

●active

When the process is done successfully, restart your machine to check if the operating system can be found.

Method 4. Rebuild damaged MBR

If none of the above methods work, it might be the damaged MBR that causes the Operating System is not found error in Windows 11/10. Thus, you need to repair it. For this task, you can run the following commands in the Command Prompt window (launch it as you do in Method 3):

●bootrec.exe /fixmbr

●bootrec.exe /fixboot

●bootrec.exe /rebuildbcd

If you dont have the Windows installation media to access the Command Prompt, you can turn to the AOMEI Partition Assistant Standard. It allows you to create a Windows PE bootable media, and rebuild the damaged MBR easily. Specific steps are listed as follows:

Download Freeware Win 11/10/8.1/8/7/XP
Secure Download

 

Step 1. Connect a blank USB drive to another Windows computer that can boot up normally. Download, install and launch the AOMEI Partition Assistant Standard on that PC as well.

Step 2. Click "Make Bootable Media" under the left "Wizards" column, a window pops up, click "Next".

Make Bootable Media

Step 3. In the next window, choose "USB boot device", then press "Proceed".

Choose USB

 

Tip: The USB drive will be formatted, so make sure to backup your important files to another location with AOMEI Backupper.

Step 4. Click "Finish" when the operation is done.

Finish

Step 5. Boot your Windows from the WinPE bootable USB you created. It contains the preinstalled AOMEI Partition Assistant Standard software. Once login, the software will run automatically.

Step 6. Right-click the system disk and choose Rebuild MBR.

Rebuild MBR

Step 7. Choose the proper MBR type for your current OS.

Choose MBR Type

Step 8. Click on Apply and Proceed to commit the pending operation.

Apply

After it is done, the damaged MBR will be rebuilt. Besides, you can also use this software to active the Windows partition.Right-click the boot partition, select "Advanced"->" Set Active Partition".

Set Active Partition

Summary

Above all are the solutions to fixing the Operating System not found error in Windows 11/10. Apparently, the AOMEI Partition Assistant Standard provides easier methods. It offers more functions to manage a disk, like disk clone, partition clone, wipe the hard drive, migrate OS to SSD, and so on. To enjoy more advanced features: convert MBR and GPT without deleting partitions, secure erase SSD, align partitions, quick partition, split partition, etc, you can upgrade to Professional.