UK Nest Heat Link help

[SSfree]

My UK Nest is having a problem with the heating not working.

My nest thermostat seems to have stopped firing my boiler for heating  in both schedule or manual mode - although the thermostat seems to be working showing the set temperature. The thermostat temperature shows up fine on both phone and yeh app - but yeh boiler does not seem to start .
The hot water is working fine and as per the set nest schedule. 

Any suggestions on what the issue is and if I can fix it? 

Please  help as I have my elderly father with me and we need the heating to start working ASAP.

[MartinJEvans]

I'm not an expert but have installed a few nest thermostats and heat links. Have you looked at the heat link box attached to your boiler? It should have a blue light on it if connected to the nest thermostat. You can usually turn your heating on at the heat link box by pressing the big round button once or twice in quick succession depending on which version you have. If the thermostat appears to be working fine and your heat link is showing connected and your hot water is ok I'd suggest the wire to the boiler asking it to fire up for heating has perhaps come out.