NTLDR is the Windows kernel. You have to either boot to a
rescue disk and repair the OS installation or, possibly try to
do an upgrade on the OS (same version as currently loaded)
and see if that can replace the file.
Another option is to boot to some other partition or disk
(floppy ie. NTFS DOS) and see if you can access the OS
installation that will not boot. If this can be done, you can
copy a version of NTLDR into the non-bootable installation.
Of course, using the upgrade, or even copying in NTLDR, method
will back out all security updates and patches so, you will have a
LONG LIST of patches to install.
Pedro