My copy of Pokémon Crystal had been on loan to a friend for years. I finally got it back, but it turned out the internal battery that retains the save and keeps the clock running had run out. So, it was time to change the battery
This post was retroactively compiled on 2024-01-19 from an old Facebook post. Nowadays, I'd get a battery that has the terminal connector legs pre-attached and just solder it in. However, this was me before I owned a soldering iron.
If you own a GBA SP, there is also a method for changing the battery without losing the saved game. And of course, there are also multiple expensive solutions for save game preservation/backup if you're so inclined.