How can i get heating back on after power cut?

I have a central heating system - the timer lost power for a long time while i was on holiday. I had to replace the batteries to get the screen back on again - since then the heating will not come back on - I still have hot water so its not the boiler (although i'm thick with things like this so not 100% sure)
The timer is a drayton digistat 2. The screen currently displays the time 12.00 and temp 20degrees but no heat from the radiators