Possible gearbox fault

Hi all, 07 plate 90,000 miles 2.0hdi egs gearbox

So driving home today, always have the car in auto, was in slow moving traffic and noticed the gearbox had gone into manual, tried selectinf neutral whilst stopped and then back to auto but it just showed 1 on the dash. Started driving again using the paddles without and issue until the traffic cleared and realised the car wouldn't go past 3rd. I pulled over and switched off and left it 5 mins, once started up again i selected auto and everything was back to normal and i made it the last 10 miles or so home.
I had no faults or errors on screen during this time so no idea what happened

Any thoughts?

Thanks