What's wrong with my Baxi Solo 2 PF 70 boiler?

We have a wall mounted Baxi Solo 2 PF 70 boiler which has just stopped heating the house and we have no hot water. It is 6.5 years old. All the usual lights are on EXCEPT the 'burner on' lights (and the 'overheat' light of course) . In the days leading up to it breaking down, We became aware of a sporadic buzzing type noise that came from the boiler itself. Any ideas what is wrong and more importantly the likely cost of replacing the broken parts?