wy won't my scenic start?

My car was driving perfectly. I stopped, went into a shop and came out - no start! Waited 15 mins. and tried - started. Drove perfect for approx. 10 miles to home. Came out half hour later and - no start.
Now it just refuses to start. It turns over but won't ignite. It's a diesel 2001 sceneic.
Please help!!!