Wont boot up, what happened? - ext4_error_find_entry

So I got my GBZ up and running, and I was enjoying an hour of Super Mario World when I hit Select+Y to save, and it froze. I waited a bit for something to happen, then I just powered 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.

*Edit - I'm using a 16gb Sandisk Ultra, class 10