no drive to front wheels

[audis8tui]

Hi guys

My wife phoned me to say she has her car stuck in the snow so off I went with a tow rope in my S8 d3.
I got stuck too as I have no four wheel drive. The rear wheels work but nothing to the front. I have the slip control warning light on the dash when wheel spinning and also the management light flashed a few times
I have just put a complete engine in the car so I know all the drive shafts are there.
What is there to check?
I gave the gearbox a service and nothing there is nothing wrong with it, the gears change great.
The only problem I have with the car is a 014 defective abs unit according to vagcom but it worked ok when slipping in the snow