Hi my name is David.
I have recently partition my harddrive to install linux,which went well I might add. Any way one day I boot my pc and found that I could no longer choose which OS to boot into. I have a "GRUB"promt: I have done some hard work and finally use chainloader to boot into my windows OS. I try to do the say to boot into Linux but had no luck. If any one has any knowldge of GRUB I would be very greatfull.

well I don't know much...but since u posed quite a while ago i'll give it a shot. linux doesn't use chainloader. Try to find a grub.conf file on the net ( a sample or s/t) and use those root images and kernel things

maybe try root = /dev/hd??

My kernel is /boot/vmlinuz.2.6.3
My root partition is in /dev/hda3

So in grub prompt you would say:
root (hd0,2) ENTER
kernel /boot/vmlinuz.2.6.3 root=/dev/hda3 ENTER
boot ENTER

Look you can boot a Linux exactly as you boot a Windows.

The trick is to have a boot loader inside the Linux.

If you Linux uses Grub you can replicate Grub by the command in bash shell (need to be root)

grub-install /dev/hdax where x is the root partition number of the Linux. Use

lilo -b /dev/hdax for Lilo as the boot loader. When that is done. These two lines in /boot/grub/menu.lst (for Grub)

title My Linux in hdax
chainloader (hd0,x-1)+1

If your Linux is in hda4 for example then x=4 and use (hd0,3) in the above.

Lilo can boot it too, also 2 lines as

other=/dev/hdax
label=Linux_hdax

All Dos, Windows, BSD, Solaris must be installed in a primary bootable partition and will have the boot loader in the root partition by default. In Linux it is an option during installation but you can always replicate it later as shown by the above.