Happened to me when my S was new. Cruise not available for a day or two then came back by itself MANY times. Longest period of "absence" was around 7 days. I could not resist calling the service centre at that time, they checked the sensors remotely, told me radar need recalibration, scheduled a check then everything came back online by itself (again). So I didn't go for the check and no recalibration was performed. "Cruise Not Available" still happened rarely afterwards, but only lasted for < 1 min each time.
It happened in my pre-facelift S too. You will learn to read the dashboard ASAP if it happens. If you see a red car in front, false alarm, as the sensors are quite sensitive. Mine reacted to a plastic bag(?) on highway before. If no red car in front of you, and the TACC / Autopilot is not cancelled, I believe that was due to another Tesla driver kept on braking at the same place numerous times before, as the TACC / Autopilot also rely on GPS info.
E.g. at this particular corner: Google 地圖
my car will brake every single time (even when it was the only car there). And it did so without the red car in display. So I am pretty damn sure someone else kept braking here in the past to create a wrong response for everyone else. And now I keep on tapping the go pedal here hoping to reverse that GPS info
Autopilot is "learning" by you using it and it running in shadow mode. I do not know whether TACC does the same. But not using it doesn't seem to be a good option. I would suggest paying attention to any deceleration, you can feel it before your speed drops too much. Tap the go pedal when you feel any deceleration AND the road ahead is clear. The car will simply go back to the set speed. Just a tap will do, no need to step on it continuously. The car will only decelerate by 1km/hr max before going back to the set speed. No one should rear-end you
