Recharging requires sunlight to fall on the solar panel. Most panels are small and produce a very low current. Anything limiting the amount of light reaching the panel can reduce the charging. A layer of dirt or dust, or shade from an overhanging plant, may be sufficient to stop the light recharging. Cleaning the panel may restore the light.
Most solar lights store electricity in two 1.5-volt nickel-cadmium rechargeable batteries. When NiCad batteries are repeatedly only partially charged, they lose the ability to hold a full charge. This may leave the batteries unable to hold enough power to illuminate the light. Batteries should be replaced after two years regardless of their performance.
The weather, the time of the year and your latitude influence the number of hours of sunshine the light receives. Days are shorter in winter, and shorter still the more northerly your latitude. The light may not receive sufficient daylight to recharge the batteries. Overcast weather may also completely prevent a panel from charging.