While the battery is likely the issue, you’re not going to know for sure until you’re able to try a different one. You likely discharged it to 0 while you left it sleeping with a flash card running.
Do you have any friends who might have a console and let you try their battery?
Well, I wasn’t suggesting that the OG colorized Gameboy game had somehow slipped under your radar. (It is, in fact, the reason the colorized hacks sometimes append “DX” to their name.) I was just reminiscing about a game from my childhood that was colorized.
While not a hack, Link’s Awakening DX was the first time I ever bought a game twice. I saved up for a summer and bought the original Gameboy bundle that came with Link’s Awakening and played through it countless times. When the DX version was released I pounced immediately and that’s the version...
The battery light should still be an indicator/warning. Depending on your brightness, power save setting, and battery health you should still have time to save and turn off once the light turns red.