Hot water first check
Firstly, we need to check it's just your hot water that's affected.
Please turn on your heating programmer and set the room thermostat to maximum. This is to make sure the boiler should light.
Here is an example of a room stat and programmer. Yours may be slightly different but the principle is the same.
Does the boiler light and stay on?
Check the radiators to make sure the chill has been taken off them.
Are your radiators getting warm?