You could try reinstalling grub onto the Ubuntu partition.
First boot into a live cd or usb stick and open a terminal.
sudo fdisk -l
which will give output like:
Disk /dev/sda: 2000.4 GB, 2000398934016 bytes
255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000a0832
Device Boot Start End Blocks Id System
/dev/sda1 * 2048 3886098431 1943048192 83 Linux
/dev/sda2 3886100478 3907028991 10464257 5 Extended
/dev/sda5 3886100480 3907028991 10464256 82 Linux swap / Solaris
If multiple disks appear look for the one with a Linux Swap to determine which is the local hard disk. The Linux partition will say 'Linux' at the last column.
Once you've found the device you need, you simply need to run
sudo grub-install /dev/###
Again replacing ### with the device file relevant to you.
This should reinstall GRUB to the Ubuntu partition and restore your ability to boot to it.