The "mouse" pad on my Acer Aspire 5920 laptop has suddenly stopped working. What can I do to mend it

You can try pressing the Fn + F7 key to activate it, if that does not help you can check in the BIOS if the touchpad is disabled.

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Make sure mouse pad utility enable in control panel.
Go to start->control panel->touchpad utility enable it.
IF still not works then reinstall touchpad driver uninstall old driver then reinstall.

you can uninstall moue pad driver from control panel.
Open add remove program in control panel then uninstall touchpad utility.Then reinstall.
Also before reinstalling driver first make sure that ouchpad option is not disable.There is touchpad utility in control panel.Also check in mouse proeprty and acceebility mouse option .
You can download driver from below lin.
Select your model and download drivers.
click here