Help! 106 not starting

[Matt]

So my neighbour has a 106, the rocker gasket had gone, so I replaced that for her. 
But she has a starting problem she wants me to fix, basically she was driving home something went bang and and then the car stopped. 
When you get in the car, turn the key, all that is heard is a very loud click from engine bay
My first thoughts was something internal, belts gone? 
Starter motor failed? 
I have no idea where to start