Don't panic! Tell us in detail what happens when you boot from the hard drive. Do you get a GRUB menu? Or just a GRUB prompt (grub>)? At what point does it say it can't find the kernel image? Once you have pinpointed where the error occurs, you are halfway to finding out what is actually causing it.
In addition, you need to tell us whether you can boot from your installation media. If you can, you can usually fix things from there.