This is a confirmed bug which affecting many other people: Bug #549727: Touchpad stops working after login. You can find in the bug comments some partial fixes, but, keep in mind, neither one is mutually agreed by everyone.
Ubuntu 13.04 will reach soon (in January) at its end of life. So you should consider to upgrade your system to 13.10 which came with a new kernel - 3.11, that could solve your problem.
Meanwhile, to solve the problem for now, first check if your touchpad is enabled. If is enabled, try to reinstall xserver-xorg-input-synaptics
.If this will not work, change the module for your touchpad (but in this last case your touchpad scroll will be dead).
I. Check if your touchpad is enabled
Open dconf Editor, navigate to org → gnome → settings-daemon → peripherals → touchpad and make sure that tauchpad-enable is enabled/ticked (check also the other options):

Check also from terminal with the following command:
synclient | grep Touchpad
If the output is different than:
TouchpadOff = 0
then your touchpad is off and you should put it on using the following command:
synclient Touchpadoff=0
If the problem still persists, then go forward to the step II.
II. Reinstall xserver-xorg-input-synaptics
To reinstall xserver-xorg-input-synaptics
package, run the following command in terminal:
sudo apt-get install --reinstall xserver-xorg-input-synaptics
After the reinstallation is finished, reboot your system. If the problem still persists, check again if your touchpad is enabled using dconf Editor, then go forward to the step III.
III. Change the module for your touchpad
If the previous solutions didn't work, do the following:
Create a file called touchpad.conf
in /etc/modprobe.d/
using the following command in terminal:
sudo -i gedit /etc/modprobe.d/touchpad.conf
Add the following line inside the touchpad.conf
file just created:
options psmouse proto=imps
Save the file and close it.
Reboot your system,
After reboot your touchpad should work normally.
dmesg
? An usb mouse works ok?