NTLDR, more commonly referred to as “NT Loader” is the name of the file that contains the boot loader for Windows. In other words, it’s the program that begins the initial Windows booting process. Without it, you can’t boot.
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The all-to-common, embarrassing reason
Now, the single most common reason for this message is actually really embarrassing, and I used to run into it all of the time myself so I know what I’m talking about – both about the problem and the embarrassment.
You left a floppy disk in the disk drive.
Now, obviously, as floppies are going the way of the Dodo, this is happening less and less. But by far, it’s the number one reason. Eject the floppy and reboot.
The issue, of course, is that the floppy doesn’t have anything to boot. Once upon a time, operating systems came on floppy disks. Older computers that have floppy drives will in fact check to see if there’s a floppy, and try to boot from it if there is one. If there’s no operating system on the floppy disk, then there’s no NT Loader and “the NT Loader missing” is the message that you get.
Now, in rarer cases, this error can occur if you’ve left a data CD in the CD drive or you’ve left a USB device hooked up to a USB connection that is somehow bootable. In both of these cases, the boot process is actually supposed to be smart enough to realize the problem and move on to boot from the hard disk instead. But I’ve definitely seen systems that aren’t quite that smart.
Ultimately, what the message really means is that the device that the system is trying to boot from doesn’t have the boot loader. Usually that means the system is trying to boot from the wrong device, as in the scenarios I just described. However, there is another scenario that’s less common but more of a concern: it’s trying to boot from the correct device, but that device has a problem.
This gets more difficult to recover from. Sometimes it’s simply a hardware problem with the hard disk. Sometimes, the boot files have accidentally been lost or overwritten. Sometimes, just random stuff happens. The one thing that’s worth trying in a case like this is to boot from your original installation media and select repair and recovery tools from the startup menu instead of choosing anything relating to installing. One of the recovery tools may very well be able to repair the damage specifically related to the boot files.
And finally, I do have to say that restoring your system to an image backup taken before the problem began should also resolve the problem, as long as it’s not hardware related.