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Check whether any lights on the keyboard show any sign of life or activity.
Recollect, whether the keyboard was dropped or otherwise subjected to mechanical shocks, anytime recently. If so, it may well have broken in some part - if no signs of life are part of the symptoms, then it may have broken the solar cell or panel (which if it is not of thin-film type solar technology, is very fragile and brittle.)
If the keyboard is 2 years old, and still not showing any sign of being 'visible' to the Solar app, then it may be 'broken', also.
Let me know what you observe and what you recollect, and I will advise you further.