PROBLEM

Upgraded Ubuntu from 15.10 to 16.04 and the touchpad now is not working, before it works normally, when you press button to toogle touchpad on your pc, it always display as disable.

SOLUTION 1

If you type

less /proc/bus/input/devices

If there is no support for your touchpad in the kernel your touchpad is detected then the problem is elsewhere. Typing

xinput

It will show X server has detected your touchpad. If your touchpad is in that list it may be possible to modify the X config.

SOLUTION 2

To change grub file,

sudo nano /etc/default/grub

Replace

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

By way of

GRUB_CMDLINE_LINUX_DEFAULT="i8042.reset quiet splash"

Save the file and then update grub using,

sudo update-grub

command. and then restart your OS,

sudo shutdown -r now

SOLUTION 3

Edit 50-synaptics.conf file :

sudo nano /usr/share/X11/xorg.conf.d/50-synaptics.conf

Add the following lines:

MatchIsTouchpad "on"
Option "TapButton1" "1"

Similar:

Section "InputClass"
    Identifier "touchpad catchall"
    Driver "synaptics"
    MatchIsTouchpad "on"
    Option "TapButton1" "1"
    Option "VertEdgeScroll" "1"

Save , exit and reboot

SOLUTION 4

Edit the grub file from,

GRUB_CMDLINE_LINUX=""

To change,

GRUB_CMDLINE_LINUX="i8042.reset i8042.nomux i8042.nopnp i8042.noloop"

SOLUTION 5

You have updated from ubuntu 14.04 to 16.04 on a Acer V5 and the touchpad stoped working,
To install,then you solve this problem,

sudo apt-get install xserver-xorg-input-synaptics