Is the actuation valve a 3-port valve? i.e. are there three pipes connected to it? If it is, and it's in the mid-position then hot water will still be reaching some of the radiators. Are all the radiators heating up or just the ones at the top of the house? If all of them are heating up, do they heat to their normal level?

Assuming that you have a pump for the radiators but that the hot water is gravity, then the valve is probably stuck in the mid-position. This could be due to either a failed pump motor (actuator) or a seized valve - or the latter causing the former. Possible also that there is a fault in the controller not sending correct on/off to valve but the first two suggestions are more likely. Need to check if the valve motor (or actuator) is receiving volts in the right places - careful, this is mains volts we're talking about here.

P.S. I'm not a heating engineer or plumber! Note the mains voltage warning!