My keyboard went dead and I suspect it might have been partly because I put a pile of DVDs between it and the window in my office, thereby restricting the light to the solar cells. There again, It might have been the age of the battery, which significantly was just over three years old - the guarantee period.
Anyway, I put the keyboard by a well-lit window for several days to no effect and then I left it out in bright sunlight for about 8 hours and then put it under the bright halogen bulb in my desk lamp over night, and lo and behold it came back to life. The first glimmer of hope was when after hours of illuminating it with bright light, the red light in the light guage momentarily flickered and that was the beginning of the recovery.
What I suggest is that you always press the button on the top right corner next to the on/off switch to ensure that there is enough light to charge the solar cells (ie the green light must come on) and don't use it unless that happens. So, it might be worth persevering.