RDH10RF, thermostat not turning radiators on?

I've got a Worcester 28CDi boiler & Siemens RDH10RF thermostat. The batteries in thermostat died so i changed them. Now it won't turn my heating on. I've got hot water but no heating. The lights on the wall mounted 'thingy' have gone off.
Pls, pls, help as winter's setting in.