I've experienced several P3B+ automtically shutdown cases, by skiping the analysis of the root cause, I wonder what is the solution to automatically bring back the Pi again while it shutdown, I mean without manually unplug and plug the power cable?
Automatically starting up after a shutdown is impossible without external hardware. With external hardware, you could e.g. put a relay on the 5V power line. Toggling that relay would simulate re-plugging the power cable.
An Arduino and a relay module would be enough for the task.