Well, this is the grub command prompt. All you have to do is...
or something similar
and then hit tab to see the possibilities. Choose the harddisk or partition where your root partition for linux resides.
or whatever your kernel is named under the /boot directory
if you chose the proper root partition or disk then your system will boot.
as for getting the system to use the grub menu when you want to boot, do some googling to find that out.