Whether caused by a virus, a new operating system or by simple mistake, being faced with an “Operating System not found” or similar error during your computer system’s boot up can be a nerve rattling experience. Assuming you have Microsoft’s Window 7 installed however, do not fear, such a boot error can often be resolved in just a few simple steps.
First, check your BIOS and hardware
In many cases, having Windows 7 fail to boot may be as simple as having your BIOS set with an incorrect boot order sequence. It can be quite common if you have more than one hard drive installed in your computer and your BIOS gets reset. Usually you can access your BIOS seconds after your computer turns on by pressing the Delete button or by pressing a specific function key. Once in the BIOS, check to see that your system drive is listed appropriately in the boot order sequence; you may need to refer to your motherboard manual for help.
Another possible reason for Windows not being detected upon start up is a hardware issue. If your BIOS is unable to detect your system drive, check to make sure all the cables are plugged in properly. If your hard drive is making an odd noise, such as a clicking sound, your hard drive may be broken. Finally, it is possible that the hard drive is having data corruption issues, which has damaged important system data, such as the Master Boot Record (MBR). If you suspect a faulty hard drive, it may be a good idea to backup and scan your hard drive for errors from another computer and possibly consider buying a replacement. Trying to repair a boot problem on a damaged drive can possibly lead to even more data loss, so backup your data before attempting anything.
The MBR and other important boot data can also be damaged by trying to install an earlier version of Windows, such as Windows XP, alongside Windows 7 and by third-party programs, such as viruses. In the case of a virus, it is recommended that you run a virus scan of the drive before attempting any repairs as otherwise it could lead to more data loss. It is further possible to achieve the appearance of damaged boot data by having the wrong drive partition set to active, which can be the outcome of an overly curious Windows user with administrative permissions.
Fixing the MBR and other start up problems in Windows 7 is most quickly accomplished by using the Windows 7 Installation DVD. If you do not have a Windows 7 Installation DVD however, you can alternatively use a Windows 7 System Recovery Disc, which we will show you how to create further down in this article. If you do not yet have either a Windows 7 Installation DVD or a recovery disc, do yourself a big favor and make a recovery disc right away to avoid any unnecessary headaches down the road.
Fixing the Master Boot Record (MBR)
Step one: Turn your computer on, booting from either your Windows 7 Installation DVD or Windows 7 System Recovery Disc. Remember, you may need to change the boot order inside your BIOS to have the your DVD drive boot first.
Step two: After the installation or recovery disc loads, if prompted, select your language settings and then continue. If you are using the installation DVD, when prompted by the following screen select Repair your computer.
Step three: The computer will take a moment now to scan itself for any Windows installations, after which you will likely be given a choice to select which installation you wish to repair. Select the appropriate Windows installation from the list and then continue. If by chance a problem is detected in one of your Windows installations at this initial stage, the system may also ask you if it can try to repair the problem automatically. It is up to you if you wish to let the system try to repair itself, but otherwise just select No.
Step four: Once you have reached the System Recovery Options screen, as shown below, you will be faced with a list of choices that can aid you in repairing a damaged Windows 7 operating system. If you wish to try the Startup Repair option first, it is often successful in automatically fixing many different start up issues, but in this article we will be using the Command Prompt option to resolve our problems manually. So, click Command Prompt to continue.
Step five: Now sitting at the command prompt, enter the following command and then press enter:
If successful, you should be greeted with the message The operation completed successfully. That’s it! Your Master Boot Record has been repaired.
While the above command does fix the MBR, and sometimes that is enough, there still might be an error with the system partition’s boot sector and Boot Configuration Data (BCD). This might occur if you have tried to install another operating system alongside Windows 7, such as Windows XP. To write a new boot sector, try the following command:
If you are still faced with your Windows 7 installation not being detected during start up, or if you wish to include more than one operating system choice to your system’s boot list, you can try the following command to rebuild your BCD:
The above command will scan all your disks for other operating systems compatible with Windows 7 and allow you to add them to your system’s boot list. If this fails, you may need to backup the old BCD folder* and create a new one in its place with the following commands:
bcdedit /export C:\BCD_Backup
attrib bcd -s -h -r
ren c:\boot\bcd bcd.old
*Some users also find simply deleting the boot folder and retrying the above steps effective at resolving boot issues, but it is not recommended.
How to change active partitions
Upon purposely changing the active partition on my system drive, I was faced with a BOOTMGR is missing error during my system’s start up that prevent Windows from starting. It is a common mistake to make when playing with partitions on a system drive and it can be a headache to solve if not prepared. To change your active partition back using the Windows 7 recovery disc or Installation DVD, follow the steps below.
Step one: Follow steps one to four in the above guide. This should take you to the Command Prompt in the Windows Recovery Environment.
Step two: Type DiskPart and then press Enter.
Step three: Type List Disk now and then press Enter. This command will list all disks attached to your computer and assign them a disk number.
Step four: Type Select Disk x, where x is the number for the disk containing the partition you wish to make active. Press Enter.
Step five: Type List Partition and then press Enter. You will now be shown a list of the partitions on the selected disk. Determine which partition you wish to make active.
Step six: Type Select Partition x, where x is the number of the partition you wish to make active.
Step seven: Now, just type Active and then press Enter. That should be it – the selected partition is now active.