Might be the immobilizer or the programing for your key. its a common fault on these (along with many others), usually going with the Remote Central Locking as well.
try the Parkers forum, the have lots of Laguna problem posts.
http://www.parkers.co.uk/advice/forum/forum.asp?FORUM_ID=8
Researching a similar problem I have, I came across a few posts where the immobilizer or key programming had gone and would let the car turn over but not start.
Needless to say, I am NOT buying another Renault
P.W
March 2008