hot water in the flat is on but the radiators are not heating up

wall thermostat turned down? motorised valve stuck or failed, check manual opener on valve

Is it a wireless thermostat, if so try changing batteries

Faulty diverter valve or faulty thermostat

It is more than likely an issue with your controls. Look for a radiator symbol in the display on the boiler when you try to put the heating on. If the symbol is there you may have a faulty diverter valve which will need somebody gas safe to look at and repair. If the radiator symbol isn't there it means your boiler isn't receiving the demand. Make sure any room thermostats are set high and if you have a wireless thermostat it may need new batteries to make it communicate with the receiver again. Most wireless stats will indicate this on the small box by boiler showing a red light.

Check your thermostat controls first could just need replacing the batteries or it's your motorised valve which diverts the source depending what's been called upon. Can check the valve itself there is a small switch which should feel stiff when you turn it to the other side and should spring back by itself. If it's easy to move it's a clear sign its faulty.