Please any effective help necessary to solve this problem is welcomed and would be appreciated. Thank you to all.

Hi and welcome to LQ.
You need to give us more details. How did it happen - what did you do? What distro are you using? How is your drive partitioned?

This is why I use the EXTLINUX bootloader. What distro you using?

the GNU GRUB manual
The GRand Unified Bootloader, version 0.97, 8 May 2005.


http://www.gnu.org/software/grub/manual/grub.pdf

Once had the same ERROR CODE 21 while extracting LIVE-CD to USB-STICK and putting GRUB-1 to MBR of HD-1 (hd0,0).
The Error Code happend when I did not insert the Pendrive just to boot into Windows.
Grub could not read the /boot/, /boot/grub/, /boot/grub/menu.lst directories and files.
It is somewhat kind of same to ERROR
15 : File not found
16 : Inconsistent filesystem structure
17 : Cannot mount selected partition
21 : Selected disk does not exist
22 : No such partition
25 : Disk read error

error 21 means that grub is looking for the grub configuration file on a disk that does not exist. Take a look at /boot/grub/grub.conf and see which disk the kernel and initrd parameters point to. They should point to the same disk where /boot is located.