If the unit is under guarantee, then take it to your dealer for service. If the unit is out of guarantee, and you wish to try and fix this yourself, the best test to undertake before sending the unit away for service is to disconnect the unit from the mains and remove the top cover. Inside, on the main board, is a small green reset button. Pressing this will clear any stored error codes that may be affecting the unit. It is also adviseable to make sure that the DAB module is firmly pushed down onto the main board. The DAB module is a small silver box, about half the size of a cigarette packet.
If this does not rectify the issue, then the unit will need to be booked in for repair through your dealer.