Boiler issue no hot water

There is a valve that opens and closes to provide heat to the radiators or to heat your water within your boiler If your heating is working and no hot water or vice versa, that valve is likely to have become stuck. my guess is your diverter valve, this valves can become faulty at anytime. Other factors that could possibly cause this issue are as follows faulty or broken diaphragms, motorized valve failing or even an airlock ( if the plumber bled your radiators after fitting and you have a combination boiler I would rule this out as your combi boiler works off the mains water pressure to your property and all air would instantaneously be pushed out during the bleeding process) You will need to contact a Gas Safe engineer to resolve the problem instead of calling plumbers out there is a difference between the two.

It sounds like it could be a faulty thermostat on the hot water side of the boiler., but it's difficult to say when it's a intermittent fault.

Their few possibilities, 1) Thermostats sensor inside boiler faulty need replacing. 2) refill system radiator air out (bleeding) 3) need servicing Most likely thermostats sensor need replacing or cleaning can’t 100% say which of above unless looked at properly by specialist and run all test. Including Combustion performance analysis to make sure it’s preforming as it should do.

Depending on exactly what boiler and system is installed it could be a number of different items that can cause this style of fault. If its a Combination boiler as stated it could be the plate to plate heat exchanger blocking up due to a sludged system or it maybe the diverter valve not operating correctly. If its a system boiler then it could possibly be a zone valve not working or again sludge in the system all things a competent gas engineer can check for you.