So if you run sudo dpkg --configure -a does it finish the upgrade? Another option to try if it will let you sudo apt-get install --fix-missing. To get into recovery mode....
Got this on the Ubuntu wiki here: https://wiki.ubuntu.com/RecoveryMode
Switch on your computer
Wait until the BIOS finishes loading (you will probably see a logo of your computer manufacturer)
The following messages will show up:
Grub loading stage1.5
Grub loading, please wait...
Press ESC to enter the menu
Quickly press the Escape key, which will bring up a boot menu. (If you see the Ubuntu logo, you've
Select the line ending with '(recovery mode)', probably the second line, something like:
Ubuntu, kernel 2.6.17-10-generic (recovery mode)
Press enter and your machine will begin the boot process.
After a few moments, your workstation should display a menu with a number of options. One of the options (you may need to scroll down to the bottom of the list) will be "Drop to root shell prompt".