Quick question lads as I've not much experience with the M47

A friends' 2005 E46 320D, serviced a few weeks ago inc new vortex breather filter.

Today after a motorway drive and stop at a crèche, it won't start. Starter turns over the engine but it won't fire.

Assuming it has not ingested a flap, what are the first things she should check?

Cheers.