2009 sedan renault megane autogearbox stark on 1 gear and wont start?

i want to knw how i can fix my car,non of mechanic around me seems to know whats the problem but the san says its the analogue sensor fault,but the problem is that they do not know where it is located.and how to fix it,second my dash board is deam,no lights on it and th car wont start?