No hot water from combi boiler?

Question:Right here goes ! Our worcester 28i combi boiler was getting hot water and central heating fine yesterday. However we are replacing out kitchen and a plumber (not corgi reg) came out this morning to take out sink out and put the caps on the ends of the hot and cold feeds..Now I fancied a bath earlier but no hot water - well tell a lie - it would get hot for about 5 seconds then it would go cold again. The boiler fires up fine - the pressure bar is just at 1.we have tried adjusting the pressure out of the main stop tap, we have tried filling the system up a bit but still no joy. The only other thing that has been done if the removal of our gas cooked the other day...but we had hot water all day yesterday but since this plumber has been we have no hot water.he hasnt done any drastic work just extended a couple of pipes and put caps on the ends of the hot and cold feeds to the sink. I know this is something really simple but we dont know what!! HELP !

Answers:
try contacting the manufacturer of the unit then a plumber