How can I get the thermostat to connect with boiler?

Hello. the thermostat RF 43-11 which controls my heating has stopped connecting with the boiler. The red light was on and flashing. I changed the batteries, hoping it was that, but now the lights are not on at all. It has always been a bit temperamental and once some one showed me how re program it by standing near the boiler and pointing the thermostat at the devise. it worked that time but I have forgotten where that info came from. Can you help at all?. Thanks