No hot water on my Baxi Combi instant HE?

I cannot get any hot water on my Baxi 80, but the central heating mode works. I can get warm water if I put the central heating on as well, but no hot water. and When I put hot water to preheat, that doesnt work either. Any idea what is wrong and how much it will cost to put right as I am retired and don't want to get ripped off. Any help/advice would be most welcome. The pressure is fine, I have checked that. Thank you. Julie