How can I fix my Siemens RDJ10RF thermostat/transmitter?

I woke up this morning to find the transmitter showing only the (incorrect) time...no temperature reading or anything else on the LCD screen. I thought it might be the batteries, so I changed them. Whilst everything else is now back on the screen, it is reading the room temperature at a constant 40 degrees (I like it warm, but not that warm), so of course the heating won't kick in. Any ideas?