If you have replaced your batteries in your room stat and also gone through the re-commissioning process and it still doesn't work it will be the dreaded capacitor fail
I have had 3 units in this week to repair, all had that fault, capacitor sorted them all out
go to ebay and enter ' drayton digistat part ' you will see the part you need, I have fixed thousands of these things :))
Dave Cardiff, December 2013