I’m not sure which version of OS X/macOS you’re using. The earlier versions to run on the early 2008 MBPs behave differently from the later, unibody models, in that if the ambient light sensors (the early 2008 models have at least two, located beneath the speaker grilles) pick up too much ambient light, they will trigger the system to deactivate the back-lit keyboard entirely.
On those earlier versions (including, but not limited to Leopard and Snow Leopard, and possibly later versions), there is a utility you can use to override that built-in, well,
override, letting you select the back-lighting level of your choice (and when to turn it off after no use), irrespective of ambient lighting levels. That utility is called
Lab Tick 0.9.4 (scroll down that page to grab the DMG installer).
Lab Tick was designed originally for the aluminium PowerBook G4s with back-lighting (as they behave the same way with respect to keyboard back-lighting being shut off altogether in a brightly-lit room as do the pre-unibody MBPs). I use this utility on my early 2008 MBP when running Snow Leopard; with the dosdude1-patched High Sierra on the same system, the utility is no longer needed as Apple, sometime along the way, between Snow Leopard and High Sierra, fixed that ambient light sensor/keyboard backlighting quirk by making pre-unibody MacBook Pros behave the same way as subsequent MacBook Pro generations.
My guess is a possible reason the keyboard back-lighting stopped working for you was because you were in a fairly bright room (not sure what the cut-off was, but I’m guessing a room in which the system records over, say, 50 lx, or maybe even 100 lx as that shut-off trigger) when it was not working, while initially the room you were in, when back-lighting worked, was somewhat darker. If you returned to it, say, in the evening, after sunset, and there wasn’t a light from above shining down directly on your keyboard/speakers, you would probably find the backlighting would work fine.
Before you install Lab Tick (which I do recommend wholeheartedly),
try this experiment:
1) set keyboard backlight brightness to maximum, so that the keys are most brightly lit.
2) Then, take your MBP into a bright room, such as a sunlit room. Expect that the keyboard backight will read as “diasbled” when you try using the F5/F6 shortcut keys to change brightness.
3) Next, whilst still in the bright/sunny room, use your hands to completely cover both speaker grilles at the same time.
If my hunch is correct, by using your thumb (on either hand) to press any key on the keyboard, this should make the keyboard back-lighting illuminate to the max brightness level you set originally in the dark(er) room.
After doing this experiment, you can then install Lab Tick.
Let us know how it goes.
