CentOS 5 Hardware :: System Won't Boot After Recent Updates?

I did a yum update to install quite a few updates to my Centos 5.3 x86_64 system. Updates included kernel and xen which required a reboot. It must be a month or so since I last updated. The updates seemed to go fine - but when the machine restarts it goes through the bios screens which seem to detect all the hard drives etc - but then the word "Grub" appears at the bottom of the bios screen - and the system hangs. It doesn't appear to respond to keyboard input. The system is using linux raid in case that is relevant.

I am able to boot a centos5.2 live CD and look at the Logical volumes - and all appears in order to my untrained eye. I have also mounted the /dev/mdo as /tmp/bootx (boot volume) and had a quick look at /tmp/bootx/grub/grub.conf and /tmp/bootx/grub/device.map and again - I can't spot any obvious problem. I note that before the updates - my default boot was "1" and now it is "0" though this appears reasonable because the xen and non-xen images seem to be listed in the opposite order to earlier updates. I have set it to boot non-xen by default - and this has always worked in the past.

I realise that the recent updates may be a coincidence - and whatever is wrong may have occurred earlier. I cannot think of anything I have done which could have broken grub though. I can probably restore my boot volume to the state it was before the updates. I should have a backup from this morning - as luck would have it. Is this a safe thing to do - or do I need to restore both boot and root to keep them consistent. I would prefer not to restore the root system unless I have to - as this would cause some data loss. However I need advice as to whether yum and other things will get confused if the boot volume is restored in isolation.