NTLDR is short for NT loader – a program loaded from the hard drive boot sector that displays the Microsoft Windows startup menu and helps Microsoft Windows load. So when it goes missing for some reason, your windows cannot load.

Though it seemed common on a routine google search, it was not so easy to fix and many forums reported that formatting hard drive was the last solution eventually.
Causes and fixes of NTLDR is Missing

Firstly I started with Microsoft Support to get an official reason for the computer error and they have the causes to it-

Corrupt NTLDR and/or NTDETECT.COM file :
1. Insert the Windows XP bootable CD into the computer.
2. When prompted to press any key to boot from the CD, press any key.
3. Once in the Windows XP setup menu press the “R” key to repair Windows.
4. Log into your Windows installation by pressing the “1″ key and pressing enter.
5. You will then be prompted for your administrator password, enter that password.
6. Copy the below two files to the root directory of the primary hard disk. In the below example we are copying these files from the CD-ROM drive letter “E”. This letter may be different on your computer.

copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\

7. Once both of these files have been successfully copied, remove the CD from the computer and reboot.

For further details refer : Microsoft