Transmission fluid drain plug fell out while driving on highway

I purchased a 2013 Hyundai accent gls. It’s an automatic. After the purchase I started driving on highway. I barely drove for 75 miles when the car gave way. Niether the accelerator nor brake were in action, the car was on cruise control cruising at 70mph when it abruptly started revving abnormally. I pulled to the side and tried to drive again manually but the car wouldn’t move. I had it towed to a service center where they found the transmission fluid drain plug was missing. They put in new plug and new fluid and car was running fine. The reason told by the mechanic was that probably someone had not tightened the drain plug properly and it came loose on highway. Everything else looked perfect near the missing plug (see image).

My question is whether any damage has happened to the transmission? (Once it stopped, I tired only once or twice to get it moving)
If yes, how much? Will it be driveable, as I’m planning to drive the car for only a year or 2.

Since it’s running fine now, and you now have fresh tranny fluid, just look up, say thanks, and enjoy life.
I’d say “you lucked out” but that isn’t entirely true. You reacted properly to keep a sudden problem from destroying your transmission. Give yourself a pat on the back (you deserve it) and keep up the good work.

There’s a pretty good chance some damage was done. Hopefully not, but if so you’ll find out soon enough. One idea, after you drive for 25 miles or so, have the transmission fluid drained out and sieved for any unusual amount of metal debris. If removing the trans pan is relatively simple, I’d probably do that too, to see if there were any broken snap rings laying about at the bottom of the pan. If no unusual amount of metal debris or parts laying in the bottom of the pan, you’d have reason to be more optimistic.

Depending on where you live you might have a 3 day return option. You might have to pay a mileage fee but this would let you out of the vehicle if that is what you want. At least show the invoice of repair to the selling dealer and they might reimburse you or give you shop credit.