There is a rocker switch to the left of the steering wheel where the SPORT mode and collision warning system buttons are located that turns off that function. It is labeled as POWER DOOR. It is pretty easy to hit accidentally with your knees. Check that it is in the ON position.
It sounds like your liftgate button has broken or a wire has come off. But check this easy thing first. You might alternately have a short in this main switch rather than the one back in your liftgate.