megane 03- electrical fault?

Had a new starter motor fitted a week ago and was cutting out on the way home from garage. However the car was very low on petrol and thought this may have been the problem, consequently after putting petrol in the problem seemed to go away. Last night, the car briefly cut out on the motorway but came back on a second later. I parked the car up, switching off the engine and then would not restart.. Theres no electrical or engine power whatsoever.. no interior lights, radio etc. only locking and unlocking doors using the card works. Does anyone have an idea what this could be?