hw not working but CH is Baxi 80E?

Basically, we had some work outside and water was switched off about 3 weeks ago, pressure was dodgy so i rang the water board and they said fiddle with the stopcock to let air out which we did and it then worked fine. don't know if this is related but for 3 days now, the CH works no problem at all, fires up and everything. The water will not fire up at all. I turn the dial to HW and when i run the boiler - nothing, not feeding the washer with HW, or bath or anything.
Circuit board was changed last year. Any ideas, we're a bit baffled as to why it works for CH and not HW.