The straightforward solution is to do exactly that! In the callback for the trigger, run whatever logic you want to end the game.
I assume you already tried exactly this, which is just exactly what you asked, so … what’s the actual question?
Is the question “how do I stop the player controller from controlling the player pawn?” If so, unpossess the pawn.
Is the question “how do I load my main menu level again?”
If so, call Open Level on the level that implements your main menu.