Car judders as it changes gear

I have noticed a few times in the last week or so that my car judders as it changes gear mainly when I put my foot down on a main road.
Then yesterday I had to call RAC out as I returned to my car, turned on the engine, put it into reverse but it just wouldn’t move. It just revved almost as if I had the handbrake on.
The engine light was also on and the gear display was flashing which it doesn’t usually.
RAC man put it into neutral and tried to roll it which he couldn’t. He kept fiddling around with the gears and eventually got it working again.
He said the problem was reading as “can’t engage second gear” he had no idea why it had happened or if it would do it again. He said it was almost as if it had just got stuck.
I’m now scared to drive it in case it happens again when I stop at traffic lights for example.
Can’t get it into the garage until end of next week. Anyone have any ideas what it might be?