In Driver Assist Options Stop Working, @verygreen wrote this concerning AP and other missing driver assist features:
When you see symptoms like this - it means your autopilot computer crashed 3 times in a row and enters the failsafe mode.
To reset it, park in a safe place and execute "car poweroff" procedure for 2 minutes (i.e. let the car stay powered off for 2 minutes), that would restart the autopilot computer and hopefully clears the problem. Leaving the car parked with nobody inside would have the same effect, only slower.