If you're on a laptop, and the trackpad stops working after upgrading, try one of these solutions in the terminal:

synclient TouchpadOff = 0

or

sudo gedit /etc/modprobe.d/touchpad.conf and in that file type "options psmouse proto=imps" (no quotes).

The second solution worked on my Lenovo T510. Credit goes to Hopper122 here http://ubuntuforums.org/showthread.php?t=1479286