Two possibilities. The first is that the calculator is fake solar powered, with a piece of plastic on top. When disassembling the calculator, it can be seen that there are no cables, so the calculator battery can only be in place. replaced. The other possibility is that the solar battery is real, but has been out of service for too long. If you don't replace it, you can also replace the built-in battery. But if it is purely solar, there is no other way. You can only change it yourself and add the battery.
A reference period: the efficiency of amorphous silicon solar panels will be significantly reduced in 3 to 5 years, and completely abandoned in 12 years. Monocrystalline silicon solar cells have a longer lifespan, but their efficiency still drops significantly after more than ten years.