Solar lights seem like a great idea when they're brand new. They're simple to install and can be moved around the yard, because they're not connected to electrical cords. Unfortunately, solar lights have a few quirks that may make them less reliable as time goes on. The biggest issue is a dirty solar panel, but that's also fairly easy to fix.
Check the Landscape
Sometimes, the light stops working simply because plants in the area have grown so much they block much of the daylight the solar panel needs to charge the device. Trim back any overgrowth, or simply move the light so its panel receives direct sunlight much of the day.
Animals or even freezing and thawing soil can also cause the light or its panel to sit at an angle instead of directly facing the sun. Reposition the light as needed to receive maximum light. While you're at it, use a soft cloth to wipe any dust and debris off the solar panel. Once the panel receives light for a couple hours, the solar light should illuminate if you put your hand over the panel.
Inspect the Light
Check the switch on the solar light to ensure it's fully in the "on" position. The switch may be on the bottom of the light capsule or under the cap of a solar landscaping light. Turn the cap counterclockwise to loosen it from the clear plastic portion of the light to access the switch on some models. Once you're sure the switch is on, set the light in an area where it receives full sun for several hours, then test it by covering the solar panel with your hand.
Moisture inside the light may also corrode the electrical connections between the panel and light. Look for signs of rust. If the device is badly corroded, you may need to replace it.
If your solar lights are on a strand, much like fairy lights, check the wire to ensure it's still in good shape. Sometimes animals chew through the cord. You may be able to fix the wire in a pinch by individually taping bare spots of each portion of the wire with electrical tape.
Refresh the Panel
In many cases, solar lights stop working because the plastic covering the solar panel gets cloudy. This is especially true for inexpensive landscaping lights. If the plastic looks cloudy after wiping it down with a damp soft cloth, apply a coat of clear nail polish once the panel dries completely. This will help the panel receive light again, which should charge the solar light so it functions properly. Allow the nail polish to dry completely, then set the solar light up so its panel will receive ample sunlight.
Check the Battery
If you've cleaned and cleared up every obvious issue and the solar light still doesn't work, it may need new batteries. The batteries are usually tucked under the solar panel assemblage. Use a tiny screwdriver to open the compartment to check the workings inside. In some cases, there may be a single AA battery inside. Replace the battery with a fresh one of the same type.