Usually this problem can be solved by checking the boot devices. If no problems found then one can try to restore the boot manager files in case they have become damaged:
NTLDR is missing
Boot: Couldn't find NTLDR. Please insert another disk.
If using Windows NT4/2000/XP then one can create a custom bootable floppy containing the boot files. Where the Boot.ini points to the boot partition, so when booting from the floppy it will load Windows NT from the boot partition and allow one to repair the boot files within Windows NT. More Info MS KB119467
Note the chance of damaged boot files is higher if the boot partition is using FAT16/FAT32, where the filesystem is easily damaged compared to NTFS. If using NTFS, then it can be a sign that the hard disk is starting to fail, and one should take a backup of important files and acquire a disk diagnostic utility from the hard disk manufacture.
More Info MS KB100312
More Info MS KB124550
More Info MS KB320397
Note if the NT Loader (Ntldr) have become compressed, then it will fail because all features of NTFS is not available during booting and give the error "NTLDR is compressed". To decompress the Ntldr use the Recovery Console and execute these two commands:
- CD \
- ATTRIB -C NTLDR