My 2014 CTS is displaying “keyless remote not detected” and the door does not open automatically like it used to.
Could it be the battery in the remote opener?
Most likely your key and car have gone out of sync.
Unlock your car with the key, put the key in the ignition, turn it on, but not starting engine, just so dash lights up. Press and hold the lock button, for about 5 seconds. Turn off and remove key. Close your door, and while sitting in the car, lock it with the remote. Then unlock it with the remote. (disclaimer: This is what you do with an Audi, so it may be different to your car, but worth a try)