how can i fix my megane 1.5 2004?

I got in my car and it started fine. stopped to get petrol, got back in my car and nothing worked! No lights on dash, nothing. RAC man said battery was fine and have since replaced starter motor. Dash now lights up for some reason but still not working. don't want to be ripped off at a dealer and if i can, i want to fix it myself. i've had it to bits so i've got a general idea where things are, just no idea why it isn't working. It's a 2004 Renault Megane Estate, 1.5 Diesel. Any suggestion would be appreciated.