10.10 cant start up

Wubi installs are particularly affected by grub updates. These updates are never security related and don't seem to add any benefit to Wubi installs so I recommend just avoiding them (packages grub-pc and grub-common). You can also lock these in Synaptic and then update-manager won't prompt you to update them.

If you have done a lot with your 10.10 install, you can probably get it booting. If not, I recommend reinstalling.

If you have made many changes and want to rescue the install, or recover the data, read on...

You can retrieve data from the Ubuntu virtual disk wubi uses with the ext2read tool (http://ext2read.blogspot.com/) - just point it at c:\ubuntu\disks\root.disk

The only workaround I am aware of for the boot problem you are experiencing is to boot an Ubuntu CD in live CD mode (Try without installing), then loop mount the root.disk and manually edit the grub.cfg file:

To do this you must identify the partition that contains the root.disk (this example assumes /dev/sda1), and then edit grub.cfg as follows:
sudo mkdir /media/win
sudo mount /dev/sda1 /media/win
sudo mount -o loop /media/win/ubuntu/disks/root.disk /mnt
sudo cp /mnt/boot/grub/grub.cfg /mnt/boot/grub/grub.cfg.copy
sudo chmod +w /mnt/boot/grub/grub.cfg
gksu gedit /mnt/boot/grub/grub.cfg

Delete all lines up to (but not including) the first line that starts with "menuentry"
Save, reboot.

Notes: the grub.cfg is not supposed to be edited by hand. It gets regenerated automatically. The workaround may have to be reapplied later. I don't know of a permanent fix, but when this happened on release 10.04.1, running "sudo update-grub" fixed it (this command will also regenerate the grub.cfg, so if it doesn't fix it, you will need to reapply the workaround).