hi there i know of renaults having abit of a immobiliser problem, this happened to a scienic in my old street and a freinds megane 1600.
they had to just keep unlocking the car then locking it for a few times and eventually it would fire up. my friend with the megane recently had the immobiliser bypassed (disconnected) and has never had a problem like it since, just something to look into. other than that it could be the fuel pump
Ian Glanister, May 2009