Why did the backlight of my MX Keys stop working?

Thanks @jayeff for your reply. I will try with the battery and see how it goes. Will update you soon.

And to answer your question, the keyboard never got wet.

@alimzahem

OK.

The question was just in case, because water can cause strange things to happen to electronics especially if there is also power connected at the time they got wet.

Cheers

But then, shouldn't connecting the keyboard to the laptop through a USB cable eliminates this theory as the keyboard is now being powered through cable and not using the battery?

@alimzahem

Depends on how the circuit is configured.

Say the battery is failing then perhaps the power from the USB can't do both, charge a failing battery and also fully power the keyboard at the same time

The above check was only to see if it is faulty because if it is a fault in the keyboard then it may be difficult to find the problem as it depends on how the lights are connected to the controller i.e. visible copper traces/wires or if they're buried in the board as sometimes keyboards are "multi-layered" and not really designed with repair in mind.

Thanks for your input on the matter. So, I guess that if it is not a battery fault, it will be hard to find a replacement for the back light layer as there are no spare parts sold from Logitech. I tried to disassemble the keyboard and reached the back light layer, there were no wires visible, the layer was kind of plastic in all. None the less, I shall hope for this to be fixed with a battery replacement. Thanks again.