tl;dr: Use a combination of a bright light and the reset technique (off ? CAPSLOCK + some keys ? on)
A few years ago while on holiday in sunny SC — a lovely place to visit, incidentally — I took the opportunity to purchase a Logitech K750 keyboard. The K750
was is a ‘solar’ keyboard- no changeable batteries, only solar cells to power it. I had been frustrated by other keyboards which needed battery changes timetabled in accordance with Sod’s Law.
They keyboard got passed on to my folks and all was well. However, since my dad passed away my mum has not used the computer as much, and so the lights in that room haven’t been on. When I went to use the computer there recently the keyboard was completely dead. Not even the red ‘sad face’ LED would light even if held close to a light source, which would normally elicit some response.
Leaving the keyboard in a decent amount of ambient light for a few days seemed to do very little to help.
Revivifying a K750
1. Turn off the keyboard.
2. While holding onto CAPS lock, keep pressing a few keys for the next 5 or more seconds.
3. Turn on the keyboard.
I did the above and held the K750 very close to a fairly bright bulb, and the LEDs sprang to life!
First the green happy face when still next to the light, then the red unhappy face when I took it out of full illumination.
So it works, for now. I was even able to type from the next room over where it sits charging beneath a mini anglepoise-type lamp.