hopefully you have rebinded the transmitter and receiver unit
http://w3.siemens.co.uk/buildingtechnologies/uk/en/building-automation/residential-controls/documentation/documents/rdh10rfsetdata.pdf
you can ring siemens helpline Phone: 0344 892 8999 to talk you through the reset/rebind procedure if these instructions look too complicated
chances are the wireless receiver unit is faulty
very common fault on this model
IF YOU ARE COMPETANT WITH ELECTRICS it is possible to link out the switch live on the receiver to get heating on
otherwise your only option is to get a heating engineer to repair/replace faulty thermostat
Q, May 2015