Worcester Greenstar 28i Junior stops working?

Hi all
I just bought a new house and the previous owner installed the Worcester Greenstar 28i Junior. I went out for holiday for three weeks and switched off the boiler by pressing the main switch.
When I came back from Holiday I switch it on. While showering I found there is no hot water. After checking the boiler, reset the boiler, the problem is still there.

I have called worcester and they said it is already out of the warranty period. The cost to look at it is 185 puls 60 reserve. A plumber came this noon and said that the problem is with the gas. The fire just does not ignite. When the boiler is switched on the flame indicator is green but went out after several seconds.

Can anyone give me some thoughts on this. Because this is the first house I bought in my life, I have no any experience about plumbing, heating systems, etc. So you help is really appreciated.