Renault scenics seem to suffer a lot from speedo faults.
There can be a number of reasons for the various faults.
If its your electronic display your best option maybe a used speedo or ringing one of the very many mobile speedo mechanics around.
If the speedo and Mileometer have stopped it can be the Speedo Drive (Located on the diff near the back of the engine) has broke. This is quite a common fault. Usually costs around 40.00 mark at the dealers.
Another reason could be the ABS sensors not working, properley or dirty. Always worth cleaning throughly first before exploring other avenues.
Unfortunatley with electric problems like this it can be down to a single wire or the whole ecu. Its a case of trial and error to find the fault. On mine it was down to a single grey wire near the headlamp which has come loose. (So check that)