What happened?

My P's gave the low battery warning, so I popped the back off in order to change the batteries. However, as soon as the back was off, it came back to life and played the little jingle it does when starting up. I was expecting the usual "Make sure you remember to set the time!" message, but instead after showing "Please wait" for longer than usual, it launched into having me set time, birthday, etc and seems like everything was wiped.

I pressed the reset button, hoping I'd get the download / reset options, but it just goes right back to setting it up as if the device had been wiped.

Is my P's dying on me? Can corrosion do this? It looks there is a tiny bit on one of the battery springs. I'm going to clean it up obviously, but is there any way for me to get my previous save data back and not have to completely reset?? 😭