If you get the "NTLDR is missing" error message, first unplug any recently installed hardware. Additionally, remove any external storage devices, and then restart the computer. If the message still appears, open the casing of your CPU, and check whether the ends of the cable connected to your hard disk are faulty or loose. If the cable is faulty, replace it with a new one, and restart the PC.
If the error message still appears, use specialized software such as Easy Recovery Essentials to fix the issue. Download the Easy Recovery Essentials image file from NeoSmart.net, and use it to create a bootable disk. When done, boot your computer from the disk you've just created.
Once the Easy Recovery Essentials window appears, choose Automated Repair, and click Continue. Once the app is done scanning your drives, identify the letter for your Windows installation from the list, select it, and click Automated Repair. Wait a few minutes for Easy Recovery Essentials to repair problems it detects with the drive. When it's done, click Restart.
Alternatively, use a Windows installation disk to replace the NTLDR and NTDETECT.COM files in your hard disk. You can also rebuild the boot.ini file from the command prompt. Afterwards, eject the disk, and restart the PC.