Why won't it start?

It could be one of many things but very difficult to speculate without being able to diagnose and perform some basic tests. Check for spark at a spark plug wire as someone cranks over the engine (it has approx 18 kiloVolts so be careful). Check for fuel pressure and fuel volume at the fuel rail while some one again cranks over the engine. If the engine cranks over sufficiently fast enough the battery and starter are OK. It could be a broken timing belt on the serious side or a blown fuse on the minor side. Get it properly checked before just replacing parts it will be cheaper in the long run.