engine was running.stopped at a junction start stop activated.went to pull away & engine died no warning lights on dash. RAC called & suspect engine failure.Anyone had this problem ? it's  a 2021 plate with 49 000 miles 

"Engine failure" could mean anything from a broken ignition wire to complete seizure of the engine, and therefore tells us nothing meaningful. I suggest you get the car to a garage to have the problem diagnosed.

Welcome Robert, thanks for joining, and sorry to hear about your issue. 
To help Cliff and I, can you tell us whether the starter motor now cranks the engine over or not?

If it does, we take it the engine doesn’t actually start now-? 
Regards,

Gareth,

sorry the garage said the crankshaft bearing has failed so new engine needed 

Ouch. That's an expensive repair, and very surprising on a 4 year old car. Has it had a hard life, or the oil levels neglected perhaps?

Thanks Robert, but it would be good if you could answer the questions asked. 
‘Crankshaft bearing has failed….’. Um, sounds strange - there are many bearings on the crankshaft. 
Have they stripped this engine to come up with that diagnosis? 
Did you have any warning indications/ noises but this failure? 
This car is not long out of warranty. Have you had it long, have you seen the service history for it if not owned it from new? 
Regards,

Gareth.,

yes audi have stripped the engine there were no warnings on the dash.car was running smooth we've had the car 3 yrs & the cars been well looked after so it's a mystery why this has happened.

Shouldn’t be, of course.
At what intervals was it serviced in the two years before you bought it, and at what intervals during your ownership - and by an Audi main dealer. 
Is the dealer/Audi going to seek some form of contribution towards the substantial cost? 
Regards,

Gareth.

not sure on the servicing prior to buying it. the car had 21000 miles on so I think it should have been served properly as we bought it from audi silverlink.

Thanks Robert,

But you are assuming it was correctly serviced before you bought it at 2 years old and at 21k miles. In terms of seeking any contribution from Audi, then assumptions are simply meaningless. You will need to wander into an Audi dealer and request a print out copy of the on-line service history - and hope it was ‘serviced strictly to the book’. 
Now on to its servicing under your ownership, as was asked - could you now tell us about that, so we can better advise you? 

51 minutes ago, Rob Rose said:

 I think it should have been served properly as we bought it from audi silverlink.

Does that mean you were not given the service history by Audi when you purchased it?