If you perform an apt-get update and upgrade and your Debian 6 based xen kernel is updated and you reboot to find your system doesn't come back up, it's likely because the default 0 / timeout lines were removed from menu.lst.
To make sure this doesn't happen either double check your menu.lst before reboot, or if you've already rebooted boot to rescue mode, mount the /dev/hd*1 partition (fdisk -l to confirm the primary partition) and modify the file.
Simple edit your /boot/grub/menu.lst file and before the first kernel line, add in:
default 0 timeout 5
The final entry will look something like this:
## ## End Default Options ## default 0 timeout 5 title Debian GNU/Linux, kernel 2.6.32-5-xen-amd64 root (hd0,0) kernel /vmlinuz-2.6.32-5-xen-amd64 root=UUID=8c7deae7-d4a0-4859-a7ca-96b67997b214 ro initrd /initrd.img-2.6.32-5-xen-amd64
Your Debian6 instance should then boot. If you are having trouble, use the console/VNC connection to connect to see the issue or contact us for help.