tyre pressure monitoring system fault

Audi A8 W12 2005 Milage 14000 From New

There Apears To Be A Fault On The Tyre Pressure Monitoring System, Will Not Initialise Or Display Tyre Pressures. The System Used To Work Fine Could Anything Require Resetting, i Did Disconnect The Battery Some Months Ago For Charging Purposes. Where Do I Start? Audi Garages Don`t Apear To Have A Clue.

Any Help Would Be Very Much Apreciated, Many Thanks Mel