My Potterton Performa 24 stopped firing (No Hot water - NoHeating)?

I have potterton performa 24 just above 2 years old combi boiler. It was working fine untill started playing up within last 3-4 months. The problem it had there is a small piece of Plastic Venturi in the middle of outlet exhaust pipe. It melt down 2-3 times. First time engineer done it and went away with £90.00 plus VAT. It melt down within 3 days..so he fixed it free. Third time he says call out charge will be there £60.00 so, I changed it my self. and it was working fine for around 2 months.

Now suddenly while having shower water went cold and fire stopped too. No flame or central heating or Hot water usage ignite any fire or sparks.

I just checked..and Venturi is fine.

What could be the problem ?

Anyone who have Potterton or Baxi knowledge to help me out.

Much appreciated.