After one of many Xen host lockups (another sorry tale in itself), one of the guest's finally gave up the ghost.
The guest in question is an OpenSuse 11.3 which (unfortunately) runs my, and several other people's, web sites.
I have managed to mount the 2 files used as virtual discs (using an excellent article here) and run e2fsck on them which doesn't report any FS errors.
I also created a new VM using these 2 files in case it was the virtual bios (or some other low level corruption) as the entire Xen host suffered this issue, so I didn't know if the issue was solely within the guest or the result of a problem at the host level.
During the boot process, the splash screen shows the following:

Starting udevd:
Loading drivers, configuring devices: ata1.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 frozen
[ 36.804405] ata1.00: failed command READ DMA
[ 36.804445] ata1.00: cmd c8/00:20:48:8c:48/00:00:00:00:00/e2 tag 0 dma 16384 in
[ 36.80447] res 40/00:01:00:00:00/00:00:00:00:00/b0 Emask 0x4 (timeout)
[ 36.804578] ata1.00: stats: { DRDY }
[ 36.961121) ata1.01: revalidation failed (errorno=-2)

This is repeated a few times, with at some stage the errorno being -5.

Eventually, I see an error message which repeats many times, stating (assuming the square brackets at the start are a time stamp, which I will leave out):

[ xxx] /etc/initscript: line 77: /etc/sysconfig/ulimit: Input/output error
[ xxx] end_request: /I/O error, dev sda, sector 59014552

This sequence is repeated many (~ 20) times, with the "offending" sector always the same.
Finally finishing with:

INIT: Id "1" respawning too fast: disable for 5 minutes

This (after 5 minutes presumably) is followed by a few repetitions of:

/etc/initscript: line 77: /etc/sysconfig/ulimit: Input/output error

and then

INIT: Id "3" respawning too fast: disable for 5 minutes
INIT: Id "6" respawning too fast: disable for 5 minutes
INIT: Id "4" respawning too fast: disable for 5 minutes

Is there any hope for the guest OS, or should I simply (ha ha - simple doesn't quite describe the process!) re-install a new guest O.S. ?

As far as I can tell, I can mount all the DomU file systems in Dom0 so I can edit config file etc. if necessary (i.e. helpful)

[ 36.804405] ata1.00: failed command READ DMA

Looks to me like disk ata1 has a hardware problem.

Thanks smallpond - I think that is a reasonable conclusion to draw also.

If there is a hardware error though, I believe it is a virtual hardware error.
The host drives all seem to be functioning just fine.
There are 3 other guests (2 Linux, 1 windows) and they boot up OK.

The fact the other VM's boot OK makes me think that it isn't the underlying host or the Xen install itself.

On the off chance it is the host disc and is because of a problem where this guest's virtual disc is located, I made a copy and created a new VM from the copy but it had the same problem when booting.
While using a copy of the original disc image isn't conclusive (it is on the same host physical drive), it definitely occupies different sectors, so I am reasonably certain there is no problem with the host's discs.

I am thinking that perhaps one or more of the guest OS drivers may have become corrupted.
I will have to do some digging and see where they are located and if I can replace them.
Perhaps this issue is less to do with Virtualisation and more to do with the guest OS itself.