This happens when the SD card gets unmounted for some reason. If the same SD card keeps unmounting, that could either be a catastrophic file system error that makes the OS throw its hands up and say “fuck it, I’m out”, or it could be a damaged SD card.

Try going into desktop mode and running a file system check on the card.

If you can catch it happening, you can go to desktop mode and collect logs (I think there’s a GUI tool for that, but if not: sudo dmesg -T > logs.txt will dump the logs in a text file). Most of those logs will be useless to you, but if you can find the moment it disconnected, you may be able to find error messages that can point you in the right directions.