Also, if you connected an external mouse, you may have automatically disabled the touchpad. Many systems are set up to this. If you disconnect the mouse, in these situations, the touchpad will be re-enabled.
There is a soft key that turns the touchpad on/off. Fn + F7 (I think) for the Aspire One. Look for the secondary functions of the buttons across the top of the keyboard.
Also, if you connected an external mouse, you may have automatically disabled the touchpad. Many systems are set up to this. If you disconnect the mouse, in these situations, the touchpad will be re-enabled.