Vaillant Ecotec 831 no hot water, but heating works fine?

I have a Vaillant ecotec plus. The heating system works fine but when you open a hot tap the boiler doesnt fire up or recognise a tap is open. A tap symbol should appear the display but nothing! All this happened when we fitted a new bathroom suite and had to shut the water off. It started to work fine for a couple of days then when we found leaks i shut the water off again but when put back on no hot water again. Can anyone help?