When the machine first froze, did you just switch it off, because you did not know how to regain control? If so, you have probably corrupted the file system and that has apparently prevented grub finding its files.
The first thing to try is to boot from the livecd, open a terminal, type su to become root, enter the root password ("root" for a standard livecd) when prompted, and then run fsck -fy /dev/sdax where ax is the letter and number of your pclos partitions. To find out which they are you can use fdisk -l (that's a lower-case 'L'). you don't need to (and can't, in fact) fsck the swap, but the / and /home or whatever you have need to be done. They must be unmounted, but should be by default with the livecd. if not, use umount /dev/sdax to unmount them.
Hopefully, once this is done, the system will now boot. If not, boot back to the livecd and use Redo MBR from the More Applications -> Configuration menu to rewrite grub and its files to the hard drive.
