From the GRUB command line, the first thing you will want to accomplish is pointing GRUB at the correct partition to find your desired kernel. If you already know which partition this is, then great- for example, on my laptop running Puppy Linux the boot partition was /dev/sda1. For reasons which are probably best left unexplored, GRUB uses a different nomenclature for block devices from the one used by Linux. In GRUB /dev/sda1 is called hd(0,1).
If you don’t know already which partition it is, then you can use the “find” command to do: find /vmlinuz or find /boot/grub/ and it will respond with the device name (for example, hd(0,1) as above). Also remember that GRUB supports tab completion: you can just type: root hd([TAB] and GRUB will automagically give you a list of partitions to choose from.
Once you’ve selected the proper “root” partition to boot from, choose your kernel:
kernel /boot/[TAB]
And GRUB will show a list of kernels in that directory. Pick one that you know works. If you aren’t sure, pick the one with the highest version number. The command line should wind up looking like this:
kernel /boot/vmlinuz root=/dev/sda1
where “vmlinux” means the kernel you chose and /dev/sda1 represents the root filesystem of your system. Since this will be passed as a boot option to the kernel, you use the Linux device-naming convention.
Once this is done, just type:
boot
and GRUB should boot your system, provided there aren’t any dependencies that I haven’t covered. If you have problems feel free to post here, to PM me, or you can google “boot from grub command line” which is how I figured out how to get my lappy working when the GRUB menu didn’t work.
I understand you don’t have an Ubuntu Live CD, but it’s very very easy to get one. You should do so as soon as you can.
Also, most USB disk images are also live systems which work as well as or better than the CD. Do you still have the USB disk you installed from? It’s good to keep such a device around, as it makes a wonderful diagnostic tool.