Van won't start now why???

What is your question?

So my boyfriend drove the van up the street to meet with his brother. It was driving just fine, then he parked and turned the van o# to meet up with his brother. When he went to leave, the van wouldn't turn back on. Why? A home garage michanic told me it was the timing chain, but another guy who woks on cars told me it doesn't sound like it's the timing chain At all!!! I need some answers please I'm desperate to find out.