no hot water?

we lost mains water and the next day we lost hot water, however the combi boiler still works on heating only. we have had a plumber look at it and he is stumpted, after consulting the manufacturers about the problem they recommended a thermostatic valve which did not work, then he tried the main PCB and still no hot water, then he replaced a part next to the pump which has a small valve on it which should go down when you are demanding hot water but it does not move, you can use a screwdriver to push it down and then you get hottish/warm water. does anybody have any idea's.