When you boot the computer, the first thing that starts up is the boot loader, usually called "Grub," which is responsible for locating the Linux operating-system on your hard-drive and loading it into memory so that it (Linux...) can start to run. Apparently, Grub can't find Linux.
In my experience, this is frequently because the boot-loader is looking in the wrong place. For instance... if you're trying to boot from a USB-stick, but the information on the stick is saying (to Grub...) that the kernel image should be found on (say) a hard drive, where it isn't.
I am not familiar with Backtrack, but perhaps this will be enough to get you started in the right direction.