you have 4 elements
the charger
the batteries
the lights
the control circuit and wiring
have you tried different batteries or charger
or tried the batteries in a different item preferably with a motor
I "fixed" a set for friend, he had put pre-charged batteries in but no lights.
The "repair" was putting the solar panel face down on the table. He had not realised that the lights would not come on if there was any light on the panel. Cost him a bottle of wine.