Siemens oven display not functioning HB 76AU.60 A

Cockroaches are notorious lovers of microwave ovens ! If they have found their way into yours expect to find damage caused by their faecal droppings which are acidic and can destroy wiring and flat wiring connections. Sometimes a good clean can revive things, but if that fails then the only remedy is to replace the control panel. That you may find to be more costly than buying a new oven.
Before removing the cover of the oven in attempt to carry out any maintenance such as replacing the Oven cavity lamp, make sure the oven is disconnected from the power point ! The voltages in a microwave oven ARE LETHAL !