Assumung you are correct in thinking that the Clock is battery-driven, you first need to find the plate at the back of the Clock, which will be covering the battery.
Remove the plate, and the Battery should be uncovered, and you will be able to discover the battery type, and buy a replacement locally at a Jeweller's shop, or at the Supermarket.
Install the new Battery, replace the back-plate, and the clock should start without any need for further action on your part.
Replace the clock on the wall !!
JadyBee, August 2009