how can i fix this problem?

I do hope somone can adise me.I have a 28 c d i rsf combi boiler.the central haeting works fine but the hot water does not come on when i run the tap nor does it ignite.when i switch the boiler off and then on the light comes on then off stright away this is when the tap is running or not running. the boiler is about 8 year old
please help if you can.thanks gary