Try to make a scematic for your self first: it makes thinking about the repair a lot easier.
Check the battery compartment and check if the moving part of the meter with the needle att to it still is in its propper place.
(If it has a meter in it......)
CdS cells seldom fail, so the trouble could be somewhere else.
If you have the diagram made up and still don't know you can send me a PM.
Most of the time these circuits are pretty simple in design.