Had the same thing last week, it was the battery even though it turned over. Before you replace it, just check the terminals and more importantly the three cables that piggy back on to the positive terminal. One of mine was loose.
Also it might just be the immobiliser confused. Lock, unlock and try again and look for the CODE light on the dash (padlock) going out.
If none of that works check you have fuel pressure, there is a valve on the fuel rails on top of the injectors, press it with a key with the ignition on it should shoot out 6 foot int air so watch your eyes and take the f@g out your mouth first
If none if that works then you need to get it looked at.
Eurocarparts have Excide 096 batteries on special at the mo, 74 amp for £55