type following commands .
step1-Grub rescue > root(hd0,0)
if this steps work then your backtrack was installed in the first partition if not then try it with
root(hd0,1) or root(hd0,2)
step2-Grub rescue > initrd <"press Tab here and you will see something that is used to boot your system">
ex initrd /vmlinuz10-30.3
step3-Grub rescue > kernel /"<press Tab here and see the Kernel version>"
ex kernel /kernel-2.35.29
if all goes fine you will be able to boot your system if not then
boot from any linux distro and try to install a new bootloader