while we are ready to boot Windows NT OS , better have to follow these steps.
Basic Intel-based computers, the system BIOS controls the initial operating system boot process.
After the initial Power On Self Test (POST) when hardware components are initialized, the system BIOS identifies the boot device.
Typically, this is a floppy disk or a hard disk. In the case of the hard disk, the BIOS reads the first physical sector on the disk, called the Master Boot Sector, and loads an image of it into memory.
The BIOS then transfers execution to that image of the Master Boot Sector.
The Master Boot Record contains the partition table and a small amount of executable code. The executable code examines the partition table and identifies the active (or bootable) partition. The Master Boot Record then finds the active partition's starting location on the disk and loads an image of its first sector, called the Boot Sector, into memory. The Master Boot Record then transfers execution to that Boot Sector image.
Whereas the Master Boot Record is generally operating system independent, the Boot Sector of the active partition is dependent on both the operating system and the file system. In the case of Windows NT and Windows NT Advanced Server, the Boot Sector is responsible for locating the executable file, NTLDR, which continues the boot process. The only disk services available to the Boot Sector code at this stage of system boot up are provided by the BIOS INT 13 interface. The Boot Sector code must be able to find NTLDR and file system data structures such as the root directory, the File Allocation Table (FAT) in the case of an MS-DOS FAT volume or the Master File Table in the case of an NTFS volume. These must be present within the area of the disk addressable by the 24-bit side, cylinder, sector structure used by the BIOS INT 13 interface and the partition table. This limits the size of the system partition to 7.8 gigabytes regardless of which file system is used.
If the error while booting is "Boot Disk Failure" then you try to run system in Safe Mode and check for error details in Event viewer .As you said someone has removed HDD from server check for IDE cable and Jumper Settings on HDD .I hope someone might have changed primary hdd to secondary while replacing hdd and viceversa.IDE connectivity and Jumper settings changes may occur in hdd boot failure.