water heater Truma Combi not working

On our first day heater and water heater worked OK.
On second day, no hot water, no cab heating.
Now checking system at home, cab heating slow but no hot water. truma controls show no faults.
have checked combi heater overheat re set button (red) all seems good.
What Am I doing wrong?
Peter.

Is your leisure battery charged up and the hab electrics switched on.
If there is not a decent 12v supply to the combi then it wont fire up

Oh yes leisure battery good, I have been connected to mains.
I have tried on mains hookup electric only, then tried gas only then combination, still no go.
switched of heater control let the heater click off try again, a little click from heater, fan spins up then shuts down. thermostat control turned right up to 5 cab chilly now.
have checked the thermal overload switch again, and frost control drain valve, all good.
now what?
Thank you for your suggestion.
I contacting the dealer now and tomorrow.

There is a reset button on the heater - see manual. Also the Truma heater will only work if it is switched on at the right hand of the two rotary switches AFTER it has 12v feed switched on. It will also NOT come back on after a mains failure, though it will survive a short 240V interruption

Thank you for reminding me about switch positions and disconnection from mains hook up.
As luck would have it I was able to take motorhome back to dealer " Dolphin motorhomes"
Their technical person tried and found? a couple off thing's.
In the end it just started working again! 
I am a plant fitter/mechanic by trade and felt it will be something simple like a loose connection or operator error.
Anyway it worked at the dealers, tried again at home, still working.
Thank you for your suggestions.