Analogue clock/timeswitch not not working on Vaillant boiler?

I've just moved in to a flat and the clock on the boiler is not working - it was until around 24 hours ago but now the time/hands etc are not moving. The heating will engage/disengage if I move the time on manually or just switch to always on but the clock itself seems to have 'stopped'. Is there a simple fix please or is it possible I inadvertently did something to stop it from working? Any advice much appreciated - thanks. PS It looks like the timeswitch 130 or very similar.