Onboard computer completely dark

Good evening together, After a long time thinking I decided to buy last month a Citroen c3 Picasso 2010 with the 1.6 vti 120 HP engine used. Fing already well at the Kies dealer that came after a day exhaust system defective. So let again there and Lambdasonde exchange. Since the last service entry was long back, I had it made last week in a friend’s workshop, complete service. Fresh oil (whereby the dealer said he made another one with it, but r just tipped oil to it), spark plugs were completely fried and the air filter hasn’t breathed freely for a long time. I also noticed that the front right, crossbar or similar is defective. Was only the rod and is now exchanged on both sides and set track. When the spark plug change compression measured, fits. Now the car is better in shot than before, but there is a little something yesterday. And indeed, the on-board computer no longer works, completely dun kel. Tacho goes without problems and it is not set to the dark mode on the radio either. Someone already had such a thing and can please tell me the solution? Radio plays, but it shows nothing at all in the display, where the missing time is the most annoying. I thank you already in advance. Greeting Matze.