When you start your heating up again after the summer you will sometimes find that one or two radiators on the system will not work. This is normally due to the TRV (thermostatic radiator valve) where the seating is stuck down.
This is an easy fix, remove the head of the TRV by unscrewing (this is not a water carrying part )exposing the pin which is top centre of valve.
Tap the pin with a small hammer lightly & the pin should pop back up if however it does not try again if it still does not move it is more than lightly that the seating on the end of the pin is stuck due to sludge in your system. sometimes if you leave the head off the pin will pop back up on its own due to the pressure of the system running.
Hope this tip is of some help on the first cold snap.