You need to repair or reinstall the operating system. Also it depends on file which is missing. Some are files are not important as others. Missing system files might indicate that you have a virus/trojan/malware on your computer.
Restore the missing or corrupt file from backup.
It's VERY hard. Only way is software.
Missing or corrupt boot sector
Mine has suddenly started doing this too. A required Microsoft Windows system directory, the Windows System32 directory is often located in either C:\Windows\System32 or C:\Winnt\system32. Often many Microsoft Windows error messages will contain the system32 directory because many of the system files Windows uses to run are stored in this directory. By itself the System32 directory will not cause any errors unless it is missing of course. If you are getting an error with a file in the System32 directory search for that file. No error message, it just pops up and has to be 'closed'
You would not be able to boot your PC, or the PC would boot on a drive other than expected. Or you might get the BIOS message that the operating system is missing.
NTLDR is a boot up (software) problem the file may be corrupt or you may be booting from the wrong disc. Check out the web page at computerhope.com below there is extensive information on troubleshooting for this problem.
not sure
Corrupt or missing driver or failing hardware.
In my case Autoexec.nt was missing from the C:\WINNT\System32 directory. Looking in the C:\WINNT\Repair directory I found that Windows had automatically made a backup copy of Autoexec.nt, so I just copied this into C:\WINNT\System32 and the problem was fixed.
A+
The boot record will not boot anymore or more precisely the message "NTLDR is missing" will be displayed. To remedy this situation boot with a copy of windows and use the "Recovery Console" in the boot menu and at the command prompt type the following: fixmbr and hit enter. Although there is a risk, there isin't much else you can do.
The part of the message that is missing is retransmitted.