I will give you one option I know would work, but it involves booting from a full install CD / ISO and using rescue mode to gain access to your boot volume and Grub. Here is what you would do to start.
- Boot from a full install CD and choose the menu option for rescue mode.
- go to your boot volume where Grub is located, usually in rescue mode your old file system is on /mnt/sysimage
- Find and edit /mnt/sysimage/boot/grub.conf to increase the timeout and unhide the menu using the following commands within grub.conf.
Grub.conf
timeout=20
#hiddenmenu
Reboot from the hdd this time and when grub shows up choose the previous working kernel and that should boot. You will be back on a previous kernel and possibly release, but you will have a system to at least back up and retry the upgrade.