If Wubi fails to reboot after upgrade, try the following things.

If the system hangs at a grub prompt, then first try to replace the c:\wubildr file with the one in c:\ubuntu\winboot\wubildr (change ‘drive’ if necessary and backup c:\wubildr first as a precaution). It appears that in many cases the upgrade is corrupting the wubildr file.

If the system fails to boot, especially with an error message,
file not found ‘loadfont’
unknown command
, then boot with a live CD, loop mount the wubi root.disk, edit the grub.cfg and remove all lines above the first “menuentry”.

Preventing the grub packages from being updated can be a permanent solution for this problem. Lock the version of the packages, grub-pc and grub-common in Synaptic Package Manager.