How do i turn back on my acer laptop touchpad after switching it off on the settings

Try the following and see if it fixes it...

1. Go into "Control Panel".
2. Click "Hardware and Sound".
3. Now click "Mouse and Touchpad".
4. Now go to "Additional mouse options".
5. The Mouse Properties window will open. In here click on your device.
6. Make sure your device is set to enabled. If it isn't then click "Enable Device".
7. Exit both that screen and "Control Panel".

Also, you might want to check for updates to your drivers, including the Synaptics drivers.

If this does not help, and an external mouse works fine, then it may well be a hardware issue.