It sounds as if the boiler is not igniting, I could be wrong but I had the same problem, you can manually relight it by turning the knob but if it is going to lock out then it is telling you something is amiss and best to get it looked at. I got so fed up with mine I got a new boiler, hopefully you will not have to go down the same route!
Really sorry to hear your problem akdasa - I can sympathise. I too am the not so proud owner of this model of boiler and continually have problems with it!
In my three year experience, I have had the blending valve changed, and the heat exchanger changed three times. The symptom of no hot water was preceeded by a significant drop in hot water pressure.......followed by drips of cold (hot) water.
Unforunately I live in a hard water area and my boiler does not like it too much.
I have been using a forum on diynot.co.uk - they have lots of helpful advice on there.
Hope you get hot water restored soon.