Thermostat not connecting to the boiler receiver?

My wall thermostat is not connecting to the receiver on the boiler. I know this as the receiver light should be green but it is red. I have followed all the instructions as regards reconnecting but it isn't working. There is an override on that I used yesterday but even that isn't working now! Boiler is fine it has just been checked, so it must be the thermostat? Have also changed batteries twice. Any advice would be much appreciated!