sudomod

So I got my GBZ up and running and I enjoyed an hour of Super Mario World, until I hit select+Y to save, and it froze, then I just powered it off. When I tried to restart, I got a bunch of errors after the Emulation Station screen (It was hard to read, but they were EXT4-fs errors, saying ext4_error_find_entry). I tried a while to get it to boot up until I eventually gave up and re-installed Retropie on the SD and it worked fine.

What happened? I can't find any info about the errors, and now I'm afraid to play any games because I know I could lose all of my progress again.

Turning off the gameboy without properly shutting down can corrupt your sd card. It appears that is what happened to you. Look for graceful shutdown mods in this forum. I believe @camble is selling a pcb to help you with this or you can make your own.