A6 (C4) alarm fob no longer talking to car

[stabes]

Last week I replaced the battery in the alarm fob on my 1994 A6 (C4). I have done this a few times over the years without issue.
This time however the fob has lost contact with the car. Pushing the button displays a bright red light but does nothing - so no deactivating the alarm that goes off every time I open the door :-)

The manual suggests sticking the fob into a slot under the steering column to put the car into override mode. This also has no effect. It's totally unresponsive.

Anyone have any idea what caused this and how to solve it. The car is now unusable :-(

Thanks,
Martin

[AllroadMass]

You can still start the car correct? Your last sentence suggests otherwise.

On my allroad, you have to reprogram the key fob with the car. I usually stick the key fob that I want to program in the driver door key hole. Turn the key to the right to lock the car. Release the key and hit the unlock button. That should do it.

[stabes]

As you say, the car is still driveable, it just sounds the alarm and hazards.
I tried your suggestion - no luck. I suspect your car is somewhat younger the mine with an entirely different system.