My Boiler will not fire up?

My halsted finest, the central heating is very tepped not hot so I can get the flat warm up and the hot water get hot the cold have to wait for about two minute the it get hot again but it does not stay hot long enough for a shower
Took mthe diverter valve and rekit it , it work for a few weeks but now the boiler has shut down and will not fire up .at all please help it is freeezing ..