The mouse pad worked just fine for days and "all of a sudden" my Acer, an Aquire R 14 (Acer Touch R3-471T), but it looks like many if not most Acers have the same issue.
The fact is that I use the Fn + F8 key to mute the speaker all the time, whenever there is an ad in a video, I really have to do that!
The Fn + F7, which is right next to the Fn + F8, is actually a key one can use to turn the Mouse Pad ON and OFF. Do it once again, Fn + F7, and the Mouse Pad will work again.
Hopefully you had a USB mouse or like me, you installed Synergy and thus could use your Linux mouse anyway. ...