Q: Trackpad gestures suddenly don't work

I have OS X, v 10.11.1. Trackpad gestures suddenly no longer work as before. No s/w updates; batteries are charged. Cannot resize open windows. Cannot drag & drop to move windows, to un-layer open windows, to move emails/folders to another location. Cannot click & drag to highlight info to be copied or deleted. The 2-finger "short swipe" does not open the Notification Center even though that gesture is selected in the trackpad configuration menu. Have tried checking/unchecking the trackpad features - and rebooting my MacBook Pro - no help.