Boot "Super GRUB2 disk" (included in UBCD):http://www.supergrubdisk.org/
It should detect your Ubuntu install, so you can boot it.
Then from within your Ubuntu install you can reinstall grub2.
I am not familiar with wubi, so I don't know if it installs grub2 in the MBR or that it lauches grub2 from the Windows boot menu.
You can install grub2 in the MBR (+ hidden sectors) or in the partition boot sector (should keep the MBR untouched.
This will install grub2 in the MBR:
$ sudo grub-install /dev/sda
This will install grub2 in the partition boot sector of the third primary partition (not sure if it is already supported in the grub2 version of Ubuntu) (first check if /dev/sda3 is your Ubuntu partition):
$ sudo grub-install /dev/sda3
Then update the configuration file (grub.cfg):
Download Ultimate Boot CD v5.0
Use Parted Magic
for handling all partitioning task: http://partedmagic.com/