Car Essentially Stalls out at a stoplight

Hello,

I have a 2011 Chevrolet Cruze, turbo (automatic). About 3 years ago I had to get a new transmission, and it the car worked fine after that. Then about a year after I was leaving the dealership after routine work and the I was stopped at a stop light and the car was essentially stuck, would not go as I was pushing on the gas. The car did not turn off or have any other signs that something else was wrong. When an officer finally arrived (I was stuck in the middle of the road) he noticed a small latch next to the gears (automatic) he used a a key to push down on the button and my car started working again. This issue hasn't presented itself again until today, essentially I was at the stop light and wouldn't accelerate and I removed the small plastic latch and pushed down on the button.

What I would like to know if what is causing this happen, and is this a serious issue? And what does this mean.

I am terrified that the fact that the car can just stop whenever.