If you haven't used it for a long time the switch contacts could have oxidised, as these cant be accessed easily without a partial dismantle you could try repeatedly operating the zoom control a few dozen times (whilst camcorder is switched off) as this might just clean the contacts into being operational again. If great care is taken, before carrying out the aforementioned you could try squirting some silicone lubrication / cleaner (as used in vehicle applications e.g:- Halfords) down the slots at the side of the control, as it might just run onto the contacts and speed the operation up.