How can i fix my boiler?

I have a wall mounted boiler.(potterton neataheat electronic),which was installed about 15 years ago.It has been a great performer in the past,but recently it has decided not to start.I know it is not the fan,as it has been tested,and is working.When i turn the boiler on,i hear the normal click,but nothing activates.Can you help in the diagnosis?.